5E: Monstrous Arthropods for Fifth Edition

[Cleon]

This thread will include giant versions of "normal" arthropods, such as Giant Spiders, and similar monstrous creepy-crawlies.

Some of these will be conversions of creatures from earlier editions of Dungeons & Dragons, but others will be entirely new monsters mostly based on arthropods that do not have official statistics in any D&D ruleset.

Finished conversions will be added to the Completed Fifth Edition Creatures Index.

There's a Want List of Monstrous Arthropods as the next post, to keep track of the monstrous arthropods we thought of converting.

For convenience, the Index of 5E Monster Arthropods will include links to the SRD arthropods that appear in the Fifth Edition Monster Manual, including some entirely fantastical species like the Ankheg and Phase Spider.

Index of 5th Edition Monstrous Arthropods

NAME​	CR​	D&D Beyond [CC]​	Enworld​
Amblypygid, Giantᴬ​	1/2​	Completed [Post]​	Completed​
Ankheg​	2​	SRD Monster​	—​
Centipede, Giant​	1/4​	SRD Monster​	—​
Crab​	0​	SRD Monster​	—​
Crab, Giant​	1/8​	SRD Monster​	—​
Drider​	6​	SRD Monster​	—​
Ettercap​	2​	SRD Monster​	—​
Fire Beetle, Giant​	0​	SRD Monster​	—​
Grasshopper, Giantᴬ³​	1/4​	Pending [Post]​	Working​
Odonatid, Giant (Dainty Giant Damselfly)​	0​	Pending [Post]​	Working​
Odonatid, Giant (Dainty Giant Damselfly Naiad)​	0​	Pending [Post]​	Working​
Odonatid, Giant (Diminutive Giant Dragonfly)​	1/8​	Pending [Post]​	Working​
Odonatid, Giant (Diminutive Giant Dragonfly Nymph)​	1/4​	Pending [Post]​	Working​
Odonatid, Giant (Giant Damselfly)​	1/2​	Pending [Post]​	Working​
Odonatid, Giant (Giant Damselfly Naiad)​	1/2​	Pending [Post]​	Working​
Odonatid, Giant (Giant Dragonfly)​	2​	Pending [Post]​	Working​
Odonatid, Giant (Giant Dragonfly Nymph)​	1​	Pending [Post]​	Working​
Odonatid, Giant (Imperial Giant Dragonfly)​	3​	Pending [Post]​	Working​
Odonatid, Giant (Imperial Giant Dragonfly Nymph)​	2​	Pending [Post]​	Working​
Odonatid, Giant (Mammoth Helicopter Damselfly)​	5​	Pending [Post]​	Working​
Variant: Mammoth Underdark Death Phantom Damselfly​	6​	Pending [Post]​	Working​
Odonatid, Giant (Mammoth Damselfly Naiad)​	4​	Pending [Post]​	Working​
Variant: Mammoth Underdark Death Phantom Naiad​	5​	Pending [Post]​	Working​
Odonatid, Giant (Titanic Tropical Dragonfly)​	7​	Pending [Post]​	Working​
Variant: Titanic Underdark Death Phantom Damselfly​	8​	Pending [Post]​	Pending​
Odonatid, Giant (Titanic Dragonfly Nymph)​	6​	Pending [Post]​	Working​
Variant: Titanic Underdark Death Phantom Naiad​	7​	Pending [Post]​	Pending​
Pedipalp, Giant (Acid Archer)ᴬᴴ​	1/4​	Completed [Post]​	Completed​
Phase Spider​	3​	SRD Monster​	—​
Schizomid, Giantᴬ​	1/8​	Completed [Post]​	Completed​
Scorpion​	0​	SRD Monster​	—​
Scorpion, Giant​	3​	SRD Monster​	—​
Solifugid, Flyweight​	1/4​	Completed [Post]​	Completed​
Solifugid, Giant​	3​	Completed [Post]​	Completed​
Solifugid, Rhagodessa​	1​	Completed [Post]​	Completed​
Spider​	0​	SRD Monster​	—​
Spider, Giant​	1​	SRD Monster​	—​
Spider, Giant Wolf Spider​	1/4​	SRD Monster​	—​
Swarm of Demonic Locusts³​	1​	Completed [Post]​	Completed​
Swarm of Giant Damselflies​	1​	Pending [Post]​	Pending​
Swarm of Insects​	1/2​	SRD Monster​	—​
—Swarm of Insects (Beetles)​	1/2​	SRD Monster​	—​
—Swarm of Insects (Centipedes)​	1/2​	SRD Monster​	—​
—Swarm of Insects (Spiders)​	1/2​	SRD Monster​	—​
—Swarm of Insects (Wasps)​	1/2​	SRD Monster​	—​
Tabanid, Giant (Giant Horsefly)ᴴᴬ​	1​	Completed [Post]​	Completed​
Trigonotarbid, Giantᴴ​	1​	Completed [Post]​	Completed​
Uropygid, Giantᴬ​	1​	Completed [Post]​	Completed​
Uropygid, Giant (Mastigoproctus)ᴬ​	3​	Completed [Post]​	Completed​
Wasp, Giant​	1/2​	SRD Monster​	—​

 

[Cleon]

This list is mainly so I remember the Monstrous Arthropods I fancied converting. It's easy to think of one and then forget about it months or years later when it's time for another conversion.

The Want List includes links for creatures with statistics that match the Index of 5E Monster Arthropods, although some of the conversions are incomplete and this isn't noted in the table.

For convenience, the Fifth Edition SRD's arthropodal monsters are included in the list with an "[SRD]" after their names.

Want List for 5th Edition Giant Arthropods

CR​	NAME​
1/2​	Amblypygid, Giantᴬ
2​	Ankheg [SRD]
#​	Belostomatid, Giant (Giant Water Bug)
1/4​	Centipede, Giant [SRD]
0​	Crab [SRD]
1/8​	Crab, Giant [SRD]
#​	Diptera (Giant Flies)
#​	Diptera (Giant Fly, Tiny)[e.g. Fruitflies]
#​	Diptera (Giant Fly, Small)[e.g. Houseflies]
#​	Diptera (Giant Maggot, Tiny)
#​	Diptera (Giant Maggot, Small)
#​	Diptera (Giant Maggot, Medium)
#​	Diptera (Giant Maggot, Large)
#​	Diptera—Calliphorid, Giant (Giant Bluebottle)
#​	Diptera—Culicoid, Giant (Dire Mosquito Female)[Anopheles & Culex?]
#​	Diptera—Culicoid, Giant (Dire Mosquito Male)
#​	Diptera—Culicoid, Giant (Dire Mosquito Wriggler)
#​	Diptera—Culicoid, Giant (Giant Mosquito)
1​	Diptera—Tabanid, Giant (Giant Horsefly)ᴴᴬ
6​	Drider [SRD]
2​	Ettercap [SRD]
0​	Fire Beetle, Giant [SRD]
1/4​	Grasshopper, Giantᴬ³
#​	Mantid, Giant (Giant Praying Mantis)
#​	Mystaran Dragonfliesᴮ
#​	Mystaran Black Dragonflyᴮ
#​	Mystaran Blue Dragonflyᴮ
#​	Mystaran Gold Dragonflyᴮ
#​	Mystaran Green Dragonflyᴮ
#​	Mystaran Red Dragonflyᴮ
#​	Mystaran White Dragonflyᴮ
#​	Mystaran Dragonfly Nymphᴮ
0​	Odonatid, Giant (Dainty Giant Damselfly)[equivalent to Griffinfly]
0​	Odonatid, Giant (Dainty Giant Damselfly Naiad)
1/8​	Odonatid, Giant (Diminutive Giant Dragonfly)
1/4​	Odonatid, Giant (Diminutive Giant Dragonfly Nymph)
1/2​	Odonatid, Giant (Giant Damselfly)
1/2​	Odonatid, Giant (Giant Damselfly Naiad)
2​	Odonatid, Giant (Giant Dragonfly)
1​	Odonatid, Giant (Giant Dragonfly Nymph)
3​	Odonatid, Giant (Imperial Giant Dragonfly)
2​	Odonatid, Giant (Imperial Giant Dragonfly Nymph)
5​	Odonatid, Giant (Mammoth Helicopter Damselfly)
4​	Odonatid, Giant (Mammoth Damselfly Naiad)
7​	Odonatid, Giant (Titanic Tropical Dragonfly)
6​	Odonatid, Giant (Titanic Dragonfly Nymph)
1/4​	Pedipalp, Giant (Acid Archer)ᴬᴴ
3​	Phase Spider [SRD]
1/8​	Schizomid, Giantᴬ
0​	Scorpion [SRD]
3​	Scorpion, Giant [SRD]
1/4​	Solifugid, Flyweight
3​	Solifugid, Giant
1​	Solifugid, Rhagodessa
0​	Spider
1​	Spider, Giant [SRD]
1/4​	Spider, Giant Wolf Spider [SRD]
1​	Swarm of Demonic Locusts³
#​	Swarm of Giant Damselflies
#​	Swarm of Giant Flies
#​	Swarm of Giant Mosquitoes
1/2​	Swarm of Insects [SRD]
1/2​	—Swarm of Insects (Beetles) [SRD]
1/2​	—Swarm of Insects (Centipedes) [SRD]
1/2​	—Swarm of Insects (Spiders) [SRD]
1/2​	—Swarm of Insects (Wasps) [SRD]
#​	Reduviid, Giant (Giant Assassin Bug)
1​	Trigonotarbid, Giantᴴ
1​	Uropygid, Giantᴬ
3​	Uropygid, Giant (Mastigoproctus)ᴬ
1/2​	Wasp, Giant [SRD]

 

[Cleon]

Odonatid, Giant (Giant Dragonfly)
Medium beast, unaligned
Armor Class 15 (natural armor)
Hit Points 27 (5d8 + 5)
Speed 10 ft., fly 60 ft. (hover)

STR​	DEX​	CON​	INT​	WIS​	CHA​
15 (+2)​	18 (+4)​	12 (+1)​	1 (–5)​	12 (+1)​	5 (–2)​

Saving Throws DEX +6
Skills Perception +3
Senses passive Perception 13
Languages —
Challenge 2 (450 XP) Proficiency Bonus +2

Evasive Flight. A giant odonatid gains advantage on Dexterity saving throws when flying and attackers have disadvantage on attack rolls against it.
 If the giant odonatid is mounted or grappling an opponent, it must make a DC 12 STR check to use Evasive Flight each time it is attacked.

Expert Aerialist. The giant odonatid has advantage on Dexterity (Acrobatics) checks when flying.

Flyby. The giant odonatid doesn't provoke opportunity attacks when it flies out of an enemy's reach.

Keen Sight. The giant odonatid has advantage on Wisdom (Perception) checks that rely on sight.

Motion Camouflage. If the giant odonatid moves at least 20 feet towards a creature and then hits it with an attack on the same turn, that target must make a Wisdom (Perception) check against a Dexterity (Acrobatics) check by the odonatid. On a failure, the target cannot use Dodging Flight, Evasive Flight, or a similar defensive ability to avoid attacks by the giant odonatid until the start of its next turn.

Actions

Multiattack. The giant odonatid makes two attacks against a Small or smaller creature. The first attack is a legs attack. If this succeeds in grappling the target, the odonatid makes a bite attack, otherwise it makes a second legs attack.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) slashing damage.

Legs. Melee Weapon Attack: +6 to hit, reach 5 ft., one Medium or smaller creature. Hit: The creature is grappled (escape DC 12). Until this grapple ends, the target is restrained, and the giant odonatid can't make legs attacks against another target.

Bonus Actions

Dashing Flight. The giant odonatid flies up to 60 feet. It cannot use Burst of Speed if it used Dodging Flight after the start of its previous turn.

Reactions

Dodging Flight. If a creature of at least Small size attempts a melee attack against a giant odonatid, or the odonatid is targeted by a ranged weapon of similar size such as a giant-hurled boulder, the insect can try to dodge out of the way as a reaction. The giant odonatid makes a Dexterity saving throw with advantage (it loses this advantage when mounted or grappling an opponent). If the saving throw beats the attack roll or DC of the attack the giant odonatid takes no damage and flies up to 30 feet.
 If a giant odonatid is carrying or grappling a creature that is too heavy for the odonatid to fly with, it must release the creature to use Dodging Flight. If the odonatid is grappling a creature light enough to carry through the air, the grappled creature can make an escape check as a free action. If it fails, the odonatid may fly away with its victim.


Description

A giant dragonfly looks identical to its tiny sized cousins except for being about 7 or 8 feet long and 250 pounds, with a 12 foot wingspan. It has a long tubular abdomen, a short sturdy thorax with four wings and six spindly legs, and an oval head that's mostly two enormous compound eyes with compact slicing mandibles tucked underneath and a pair of short antennae on its brow. The body is brilliantly colored in stripes and spots, most often in metallic greens, blues and golds. Dragonflies have translucent, dark-veined wings with a short strip of color on the leading edge near each wingtip (the pterostigmata). The rest of the wing is generally clear and colorless but it may be pigmented. Most normal dragonflies have clear wings, but almost all giant dragonflies have colored wings; the majority are uniform pale amber but some have stripes or patches of bright color, most commonly russet, scarlet or peacock blue. The most colorful giant dragonfly's wings rival the beauty of a stained glass window.
 Giant dragonfly skin can be transformed into gorgeous leather and is prized by some societies for use in apparel, accessories and decorations. The wings are sometimes used as window panes or fans.
 Aerial Hunters. A dragonfly is an amazingly fast and agile flier, able to move in any direction–including backwards–and can hover as if it's nailed to the air. Their six legs are not built to move quickly on land, but are used for perching and to form a "basket" for catching prey. Natural dragonflies hunt on the wing, preying upon flying insects smaller than themselves. The dragonfly snatches their prey in its leg basket and bites then either devours them in the air or lands on a perch to feed (most often when its meal is inconveniently heavy). The monstrous giant variety preys on creatures up to its own size, and unlike normal dragonflies it will hunt creatures on the ground as well as in the air. The commonest prey are smaller creatures (e.g. a baboon, giant fire beetle, rat, or an unlucky halfling), and it particularly focuses on flying prey (e.g. a bat, eagle or giant grasshopper).
 Like most giant insects, a giant dragonfly operates entirely on instinct. If the prey looks small and light enough to carry through the air, the giant dragonfly instinctively uses the snatch-and-bite tactic. If the target looks too big and heavy for this tactic, a giant dragonfly repeatedly darts in to bite and then flies out of its victim's reach until it prey falls or the insect receives a significant injury. It automatically flees if approached by any creature bigger than itself, being "programmed" to identify such as potential predators. While a natural dragonfly is exclusively a solitary predator, small groups of giant dragonflies will sometimes attack together and share the kill. This behavior mainly occurs among dragonflies that live in areas with abundant Medium prey. The attacking flight of dragonflies numbers up to a half-dozen or so and does use coordinated tactics like a pack of wolves or lions. Their cooperation is limited to not eating each other.
 Flyers of Lakes and Marshes. Like their normal kin, giant dragonflies congregate over freshwater and spend most of their time hunting for food and breeding. Most giant dragonflies prefer water that's still or slow flowing with abundant aquatic vegetation; typically a lake, pond, marsh or sluggish river. They are only active during the day, sleeping on perches at night. A giant dragonfly often has favorite perches that overlook a good hunting and/or breeding area, which they will defend vigorously from rival dragonflies. Males are way more territorial than females. Dragonflies sometimes fly great distances in search of new bodies of water to claim as their own.
 Like their normal kin, a giant dragonfly larva is an aquatic creature called a nymph or naiad (see Giant Dragonfly Nymph for details). Each species of giant dragonfly can be extremely fussy about what type of water they breed in. The temperature, chemistry, depth, vegetation and current must match the female's exacting standards. The dragonflies mate in midair, often after ferocious aerial jousts between rival males, then the female finds a suitable section of water to deposit her eggs. A female giant dragonfly lays her eggs in multiple clutches, some as small as one or two eggs, which she may scatter around as many good breeding spots as she can find. Some giant dragonflies will lay eggs in surprisingly small bodies of water barely large enough for a single full-grown nymph. If the food supply and other conditions are right, some giant nymphs could mature in a large barrel of water (e.g. a tun barrel of 210 to 252 gallons) or a pool with similar dimensions.

VARIANT: GIANT TROPICAL DAMSELFLY
While a standard giant damselfly is a Small beast some tropical species grow much bigger (see Giant Damselfly for details). A giant tropical damselfly averages 10 feet in length, with a wingspan of 10 to 14 feet. It has a giant dragonfly's statistics except for being slower (fly 50 ft., Dashing Flight 30 ft., Dodging Flight 30 ft.). Giant tropical damselfly naiads are as big and dangerous as giant dragonfly nymphs.

VARIANT: TENEBROUS & UNDERDARK GIANT HELICOPTER DAMSELFLIES
The majority of monstrous helicopter damselflies can see in the dark far better than any natural damselfly, with tenebrous species that hunt in the gloom of forests and nighttime and even subterranean varieties native to the underdark (see Mammoth Helicopter Damselfly for details, under Tenebrous Helicopter Damselflies and Underdark Helicopter Damselflies). A giant damselfly of these variants uses a giant dragonfly's statistics except for having better senses—the tenebrous variety has blindsight 20 ft., darkvision 60 ft.; the underdark variety, dubbed a ghostwing, has blindsight 40 ft., darkvision 90 ft.


GIANT ODONATID SIZES & VARIANTS
The giant odonatid entries and their subtypes are presented below, in order of increasing size. The lengths and weights of quoted in the monster entries are averages for each category, and individuals will vary depending on their species and growth history. Particularly elongated and slender specimens could be up to 25% longer for damselflies or 33% for dragonflies while weighing the same, while extraordinarily stocky ones might be as little as 80% the quoted lengths. Wingspan tends to stay the same, although some odonatids have broader, shorter wings than others, especially among dragonflies. In general, male and female dragonflies are roughly the same size, while damselflies show a tendency for males to be somewhat larger than females. The most important period of growth is the aquatic larval stage, as the biggest adults emerge from the biggest naiads or nymphs. Adult odonatids do not molt so have no way to grow bigger, although they can gain weight by eating well, or lose body mass from malnutrition and the stresses of breeding or migration.
 The larva of an odonatid is called a nymph or naiad. The terms are interchangeable, but for consistency these entries use naiad for a damselfly larva and nymph for a dragonfly larva.

Dainty Giant Damselfly (Dragonfly; Tenebrous & Underdark Helicopters)
Dainty Giant Damselfly Naiad* (Dragonfly Nymph; Terrestrial Larvae; Tenebrous & Underdark Naiads)
Diminutive Giant Dragonfly (Damselfly; Tenebrous & Underdark Helicopters)
Diminutive Giant Dragonfly Nymph (Damselfly Naiad*; Terrestrial Larvae; Tenebrous & Underdark Naiads)
Giant Damselfly (Dragonfly; Tenebrous & Underdark Helicopters)
Giant Damselfly Naiad* (Dragonfly Nymph; Terrestrial Larvae; Tenebrous & Underdark Naiads)
Giant Dragonfly (Tropical Damselfly; Tenebrous & Underdark Helicopters)
Giant Dragonfly Nymph (Damselfly Naiad*; Terrestrial Larvae; Tenebrous & Underdark Naiads)
 Terrestrial Giant Odonatid Larvae
Imperial Giant Dragonfly (Tropical Damselfly; Tenebrous & Underdark Helicopters)
Imperial Giant Dragonfly Nymph (Damselfly Naiad*; Terrestrial Larvae; Tenebrous & Underdark Naiads)
Mammoth Helicopter Damselfly (Tropical Damselfly; Dragonfly; Underdark Helicopters)
 Tenebrous Helicopter Damselflies
 Underdark Helicopter Damselflies
Mammoth Damselfly Naiad* (Dragonfly Nymph; Terrestrial Larvae; Tenebrous & Underdark Naiads)
Titanic Tropical Dragonfly (Tropical Damselfly; Tenebrous & Underdark Helicopters)
Titanic Dragonfly Nymph (Damselfly Naiad*; Terrestrial Larva & Tenebrous, Underdark Naiads)
* Note the standard Mammoth Helicopter Damselfly is a Tenebrous Helicopter Damselfly, a Helicopter Damselfly Naiad is usually a Tenebrous Giant Damselfly Naiad rather than a Giant Damselfly Naiad.

Swarm of Giant Damselflies (Dragonfly; Tenebrous & Underdark Helicopters)​

(Monster designed by Casimir Liber and Cleon on Enworld.org's General Monster Talk Creature Catalog Forum; based on the giant dragonfly that first appeared in EX2 The Land Beyond the Magic Mirror (1983) by Gary Gygax.)

 

[Cleon]

…and placeholder is planted.

Will need to stat up their Nymphs too, possibly at different sizes. The AD&D version has Dragonfly Larva smaller than the adults, but they actually grow slightly bigger before metamorphosing into the winged adult form. Also, a half-grown Giant Dragonfly Nymph would still attack an average humanoid in the water, so will arguably need stats. Heck, one that's less than half size might be game. So maybe stat up Small, Medium and Large naiads? They're probably Tiny when they hatch from the egg, but those would be too small to pose any threat to humanoids.

The other arthropods I was considering are:

Pseudostigmatid, Giant (Giant Forest Damselfly) the superbig "helicopter" damselflies.

Zygopterid, Giant (Giant Damselfly): a giant regular damselfly that's less powerful than a Dragonfly. Less of a priority.

Reduviid, Giant (Giant Assassin Bug).

Belostomatid, Giant (Giant Water Bug) was originally thinking of doing this as a variant entry of the Giant Assassin Bug, but while both families are Heteroptera they aren't that similar.

Mantid, Giant (Giant Praying Mantis) since I can't not include them.

Incidentally, in BECMI a dragonfly is literally a Dragon Fly. They're magical hybrids of chromatic dragons and anisoptera that look like giant-sized dragonflies with a somewhat draconic head.

Yes, obviously they have Breath Weapons, and they can use them AND bite every combat round.

Shall we do those too?

 

[Cleon]

Odonatid, Giant (Giant Dragonfly Nymph)
Medium beast, unaligned
Armor Class 15 (natural armor)
Hit Points 39 (6d8 + 12)
Speed 30 ft., swim 10 ft.

STR​	DEX​	CON​	INT​	WIS​	CHA​
16 (+3)​	15 (+2)​	14 (+2)​	1 (–5)​	12 (+1)​	5 (–2)​

Skills Stealth +6
Senses blindsight 20 ft., passive Perception 11
Languages —
Challenge 1 (200 XP) Proficiency Bonus +2

Ambusher. In the first round of a combat, the giant odonatid has advantage on attack rolls against any creature it surprised.

Actions

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (2d4 + 3) slashing damage and the target is grappled (escape DC 13). Until this grapple ends, the target is restrained, and the giant odonatid can't bite another target.

Death Mask (Recharge 4–6). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (2d4 + 3) piercing damage. If the target is a Large or smaller creature, the target is grappled (escape DC 13) and must make a DC 13 Strength saving throw. On a failure, the target is knocked prone, and the giant odonatid can drag the target closer and make one bite attack against it as a bonus action. Until this grapple ends, the odonatid cannot use its Death Mask attack again.

Jet Dash (Recharge 5–6). The dragonfly nymph swims up to 80 feet.


Description

A giant dragonfly nymph is a sturdy insect colored the dull gray-brown of mud or decayed plants. A dragonfly nymph is shorter and thicker than an adult dragonfly and lives in freshwater (usually—see Terrestrial Larva, below). A fully grown giant dragonfly nymph is typically 4½ to 6 feet long, although particularly stocky species can be 4 feet long or even less, with broad or thick bodies that may be somewhat flattened. The nymph has a globular head with large compound eyes similar to an adult dragonfly, although the eyes aren't quite as big, with a "mask" of smooth chitin covering the lower half of its face. A dragonfly nymph has fairly long legs and prefers to move slowly and deliberately, sometimes hesitating before each step. On its back are a pair of wing-buds, sometimes called wing-sheaths, that become larger as the nymph develops.
 Masked Killer. A giant odonatid larva may be ugly to some but might not look dangerous, as it lacks obvious jaws or claws. This is far from the truth as odonatid larvae are all voracious predators. The "mask" covering the larva's face has a concealed pair of hooked jaws, and is the front of a lip-like organ folded under the insect's head called a labium. This organ can unfold with lightning speed to seize prey, which are then pulled in to be devoured. A giant dragonfly nymph can be particularly pugnacious, attacking prey several times its own weight. Odonatid larvae are ambush predators who wait for prey to blunder into them. If one senses a nearby meal, the larva may stalk it, inching closer until the target is within reach of its labium. A larva always start with a Death Mask attack and, if successful, tries to kill and eat its grappled victim with bite attacks. If they fail to grab their target, the larva will continue to press the attack as long as its opponent stays within range, but a giant nymph naiad or almost never chases prey that flees from its reach.
 Lake Panthers. Most giant dragonfly nymphs live on the bottom of a lake or swamp, hiding under mud or vegetation or lurking in some crevice. Adult giant dragonflies tend to produce far more eggs than a body of water could support. While natural dragonfly nymphs tend to avoid the surface, the giant variety sometimes hunt like crocodiles, hiding near the water's edge to attack land creatures that come to drink.
 While monstrous giant dragonfly adults will sometimes join forces when attacking, giant odonatid larvae are always solitary. A cluster of dragonfly nymphs will avoid each other, with smaller nymphs hiding at the edges of the pond to avoid being eaten by the larger ones. Two nymphs of roughly the same size may threaten each other with displays of their legs, mask or jaws to establish or defend their territory (especially if both are the same species) but fights to the death are not uncommon.
 Even a small pond may contain multiple giant odonatid larvae, possibly of several species. In tight confines, the biggest and strongest eat their smaller rivals. This is often whichever giant odonatid larva hatched first rather than whichever one could grow the biggest. Small giant dragonfly nymphs can (and will) eat Tiny imperial dragonfly nymphs that would be Large and eat them if given time to grow.
 Life in the Water. Like most insects, giant odonatid larvae grow in stages called instars, molting its exoskeleton each time it goes to the next, larger instar. Most odonatid instars look similar apart from their size, but the last few start to develop obvious "wing buds" on the thorax. The speed and number of instars it takes to reach adulthood varies depending on species and growth conditions, with temperature and food supply being the most important.
 Once the giant dragonfly nymph reaches full size, which can take years, the final instar climbs from the water and metamorphoses into an adult in its final molt, then pumps blood into the veins of its new wings to spread them out. This transformation can take several hours, during which the giant dragonfly is completely helpless until its new wings and exoskeleton harden. They are so vulnerable, most giant odonatids try to hide somewhere before metamorphosing, and they might wait for the cover of darkness before emerging from the water. It is mostly only tropical species that emerge at night, since their adult bodies harden more quickly in hot weather and the nights are too cold in temperate climes. The newly formed dragonfly, called a teneral, then flies off. Tenerals are pale or dull, it takes the dragonfly a few days to gain a mature adult's bright colors.

VARIANT: GIANT TROPICAL DAMSELFLY NAIAD
This is the larva of a Medium giant damselfly, often a tropical species (see Giant Dragonfly for details). A giant tropical damselfly naiad averages 7½ feet long and uses a giant dragonfly nymph's statistics, except it is faster (30 ft., swim 30 ft.) and loses the nymph's Jet Dash action option.

VARIANT: GIANT TENEBROUS & UNDERDARK DAMSELFLY NAIADS
These are larvae of monstrous damselflies adapted to live in dark environments (see Mammoth Helicopter Damselfly for details, under Tenebrous Helicopter Damselflies and Underdark Helicopter Damselflies). A giant tenebrous damselfly naiad has blindsight 20 ft. and darkvision 60 ft.

A giant underdark damselfly naiad uses a giant dragonfly nymph's statistics, except it moves as well on land as water (30 ft., climb 30 ft., swim 30 ft.), has blindsight 40 ft. and darkvision 90 ft., and gains the following additional trait.
 Amphibious. The underdark damselfly naiad can breathe air and water.

VARIANT: GIANT TERRESTRIAL DAMSELFLY & DRAGONFLY LARVAE
This is a giant tropical damselfly naiad or giant dragonfly nymph adapted to live on land, although It cannot survive dry conditions for long (see below for details, under Terrestrial Giant Odonatid Larvae). A giant terrestrial odonatid larva uses a giant dragonfly nymph's statistics, except it moves better on land than water (30 ft., climb 20 ft., swim 10 ft.), loses the Jet Dash action option if it's a dragonfly nymph, and gains the following additional trait.
 Limited Amphibiousness. The terrestrial odonatid larva can breathe air and water, but it needs to be drenched in water at least once every 4 hours to avoid suffocating.


TERRESTRIAL GIANT ODONATID LARVAE
A few naiads and nymphs are able to breathe air and spend some or most of their time on land. Even the normal aquatic larvae have to breathe air at least once, in order to survive out of water long enough to transform into adults. Some of the terrestrial air-breathing species just venture out earlier, so spend some of their later instars hunting for prey out of the water. Others spend pretty much their entire larval lives on land. These terrestrial species need to keep themselves damp to stay alive for they are unable to breathe if they dry out. Most therefore keep to the soggy banks of fast-flowing streams and rivers. For example, the giant waterfall damsel (also known as the cascade damselfly, Thaumatoneura inopinata) lives in the constant spray of waterfalls. Its naiads hatch from eggs laid in damp moss on the cliffs beside the falls, and live among the wet rocks and mist-dampened roots. Other species could live in damp undergrowth, such as the deep leaf litter of a tropical jungle.
 While very few species of natural odonatids have terrestrial larvae, it's more common among the giant monster versions. Possibly they find it easier to breathe air than the normal-sized insects, or their size makes them far more resistant to dehydration. As well as the cascades and damp undergrowth mentioned above, these giant terrestrial odonatids are also found in wet cave systems. These troglodytic species are almost all tenebrous damselflies able to see in the dark, and these giant odonatids may be the original stock that the underdark damselflies evolved from (see Tenebrous & Underdark Damselfly Naiads above for an example of such creatures).

(Monster designed by Casimir Liber and Cleon on Enworld.org's General Monster Talk Creature Catalog Forum; based on the giant dragonfly nymph that first appeared in EX2 The Land Beyond the Magic Mirror (1983) by Gary Gygax.)

 

[Cleon]

Odonatid, Giant (Giant Damselfly)
Small beast, unaligned
Armor Class 14
Hit Points 14 (4d6)
Speed 10 ft., fly 50 ft. (hover)

STR​	DEX​	CON​	INT​	WIS​	CHA​
10 (+0)​	18 (+4)​	11 (+0)​	1 (–5)​	12 (+1)​	5 (–2)​

Saving Throws DEX +6
Skills Perception +3
Senses passive Perception 13
Languages —
Challenge 1 (200 XP) Proficiency Bonus +2

Evasive Flight. A giant odonatid gains advantage on Dexterity saving throws when flying and attackers have disadvantage on attack rolls against it.
 If the giant odonatid is mounted or grappling an opponent, it must make a DC 12 STR check to use Evasive Flight each time it is attacked.

Expert Aerialist. The giant odonatid has advantage on Dexterity (Acrobatics) checks when flying.

Flyby. The giant odonatid doesn't provoke opportunity attacks when it flies out of an enemy's reach.

Keen Sight. The giant odonatid has advantage on Wisdom (Perception) checks that rely on sight.

Motion Camouflage. If the giant odonatid moves at least 20 feet towards a creature and then hits it with an attack on the same turn, that target must make a Wisdom (Perception) check against a Dexterity (Acrobatics) check by the odonatid. On a failure, the target cannot use Dodging Flight, Evasive Flight, or a similar defensive ability to avoid attacks by the giant odonatid until the start of its next turn.

Actions

Multiattack. The giant odonatid makes two attacks against a Tiny or smaller creature. The first attack is a legs attack. If this succeeds in grappling the target, the odonatid makes a bite attack, otherwise it makes a second legs attack.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d4 + 4) slashing damage.

Legs. Melee Weapon Attack: +6 to hit, reach 5 ft., one Small or smaller creature. Hit: The creature is grappled (escape DC 10). Until this grapple ends, the target is restrained, and the giant odonatid can't make legs attacks against another target.

Bonus Actions

Dashing Flight. The giant odonatid flies up to 30 feet. It cannot use Burst of Speed if it used Dodging Flight after the start of its previous turn.

Reactions

Dodging Flight. If a creature of at least Small size attempts a melee attack against a giant odonatid, or the odonatid is targeted by a ranged weapon of similar size such as a giant-hurled boulder, the insect can try to dodge out of the way as a reaction. The giant odonatid makes a Dexterity saving throw with advantage (it loses this advantage when mounted or grappling an opponent). If the saving throw beats the attack roll or DC of the attack the giant odonatid takes no damage and flies up to 20 feet.
 If a giant odonatid is carrying or grappling a creature that is too heavy for the odonatid to fly with, it must release the creature to use Dodging Flight. If the odonatid is grappling a creature light enough to carry through the air, the grappled creature can make an escape check as a free action. If it fails, the odonatid may fly away with its victim.


Description

A giant damselfly resembles its relative the giant dragonfly (see Giant Dragonfly for details), but is smaller and more slender, with an abdomen that's proportionally longer and thinner and eyes that cover less of the face. The two insects are easy to distinguish when perched, for a damselfly folds its wings back vertically above its body when resting while a dragonfly spreads its wings wide and holds them flat. A standard giant damselfly is about 6 feet long, has a 6 to 8 foot wingspan, and weighs 50 pounds; shorter and much lighter than an average giant dragonfly. However, a few species of giant damselfly are much bigger and have statistics that compare to a giant dragonfly or imperial giant dragonfly and the largest, the mammoth helicopter damselfly, is so enormous they dwarf all other giant odonatids (see Giant Dragonfly, Imperial Giant Dragonfly and Mammoth Helicopter Damselfly for details).
 Flying Predators. Giant damselflies are generally similar to dragonflies in habits. Unlike giant dragonflies, most giant damselfly take after their normal-sized relatives and only hunt in the air, so pose little threat to creatures on the ground even if they're small and weak enough to be easy prey. A giant damselfly will eat any flying creature it can catch, most often Tiny creatures such as bats, birds (including hawks, owls and ravens), flying snakes or giant fruit flies. Some areas welcome giant damselflies because they eat serious pests such as stirges and giant mosquitoes, which are also a favorite food for giant damselfly naiads (see Giant Mosquito for details).
 Shimmers of Damselflies. Giant damselflies tend to prefer faster-flowing water than dragonflies, although there is great variation between species with some that thrive in stagnant or brackish water and even one whose young live in seawater. Like dragonfly larvae, damselfly larvae are voracious aquatic predators (see Giant Damselfly Naiad for details).

VARIANT: SMALL GIANT DRAGONFLY
Some varieties of giant dragonfly are the same size as an average giant damselfly, although most are longer and far heavier. A small giant dragonfly is 4 to 4½ feet long, with a wingspan around 7 feet. It has a giant damselfly's statistics except for being faster (fly 60 ft., Dashing Flight 60 ft., Dodging Flight 30 ft.).

VARIANT: SMALL GIANT TENEBROUS & UNDERDARK HELICOPTER DAMSELFLIES
The majority of monstrous helicopter damselflies can see in the dark far better than any natural damselfly, with tenebrous species that hunt in the gloom of forests and nighttime and even subterranean varieties native to the underdark (see Mammoth Helicopter Damselfly for details, under Tenebrous Helicopter Damselflies and Underdark Helicopter Damselflies). A small giant damselfly of these variants uses a giant damselfly's statistics except for having better senses—the tenebrous variety has blindsight 20 ft., darkvision 60 ft.; the underdark variety, dubbed a ghostwing, has blindsight 40 ft., darkvision 90 ft.

(Monster designed by Casimir Liber and Cleon on Enworld.org's General Monster Talk Creature Catalog Forum; inspired by the giant dragonfly that debuted in EX2 The Land Beyond the Magic Mirror (1983) by Gary Gygax.)

 

[Cleon]

Odonatid, Giant (Giant Damselfly Naiad)
Small beast, unaligned
Armor Class 14 (natural armor)
Hit Points 18 (4d6 + 4)
Speed 30 ft., swim 30 ft.

STR​	DEX​	CON​	INT​	WIS​	CHA​
12 (+1)​	15 (+2)​	13 (+1)​	1 (–5)​	12 (+1)​	5 (–2)​

Skills Stealth +6
Senses blindsight 20 ft., passive Perception 11
Languages —
Challenge 1/2 (100 XP) Proficiency Bonus +2

Ambusher. In the first round of a combat, the giant odonatid has advantage on attack rolls against any creature it surprised.

Actions

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) slashing damage and the target is grappled (escape DC 11). Until this grapple ends, the target is restrained, and the giant odonatid can't bite another target.

Death Mask (Recharge 4–6). Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) piercing damage. If the target is a Medium or smaller creature, the target is grappled (escape DC 11) and must make a DC 11 Strength saving throw. On a failure, the target is knocked prone, and the giant odonatid can drag the target closer and make one bite attack against it as a bonus action. Until this grapple ends, the odonatid cannot use its Death Mask attack again.


Description

A typical giant damselfly naiad or nymph is an aquatic insect around 5 feet long, usually green or brown in color. They have wing-nubs on their backs and a "mask" covering the lower face like giant dragonfly larvae, but are far slimmer in build, with flexible streamlined bodies better shaped for swimming (see Giant Dragonfly Nymph for details). A damselfly naiad does not have internal gills like a dragonfly nymph, but breathes with external gills at the tip of its abdomen that usually form a three-lobed trident that also acts as a tail fin.
 A naiad of this size could be the larva of a giant damselfly that is nearly fully grown or in its final instar (see Giant Damselfly), a half-grown juvenile of a man-sized giant odonatid (see Giant Dragonfly) or an earlier instar of an even bigger damselfly, likely one of the giant tropical helicopters (see Imperial Giant Dragonfly, Mammoth Helicopter Damselfly, or Titanic Tropical Dragonfly).
 Lurkers in Lake Weed. Damselfly naiads hatch from eggs. Damselflies and dragonflies have a range of egg-laying strategies. Some species lay eggs above the water, and the larvae drop in when they hatch. The majority lay in water, mostly on water plants—some stick their eggs to the plant, others cut a hole with a blade on their abdomen tip and hide the egg inside. In either case, the female odonatid must lower its abdomen in the water, often entirely submerging itself during its egg-laying. The remaining species mostly just hover and dip their abdomen in water to drop their eggs, which allows them to quickly scattering a clutch over a wide area. Some tropical giant odonatids leave eggs in the water-filled hollows of plants, called phytotelmata (see Mammoth Helicopter Damselfly for details).
 While dragonfly nymphs hunt on the bottom, damselflies prefer to live nearer the surface. They tend to hide in underwater vegetation and ambush passing prey. In general, damselfly naiads are reluctant to attack large prey that dragonfly larvae would not hesitate to ambush, and naiads often avoid hunting creatures their own size or larger. There are damselflies that breed in most kinds of freshwater, provided it is clean enough. One natural damselfly even breeds in seawater, with naiads that live in rock pools on the seashore. There are multiple monstrous giant species that can do the same, breeding in sheltered reefs and saltwater bays. Other monster damselflies have naiads that live on the land or are adapted to even more extreme habitats, such as the eternal darkness of caves or bubbling pools of water so rich in minerals it is usually only brine flies and bacteria that can tolerate it. Some of these varieties are described below (see Giant Damselfly for the adult forms).

VARIANT: SMALL GIANT DRAGONFLY NYMPH
This is the larva of a Small giant dragonfly (see Giant Damselfly for details). A small giant dragonfly nymph is around 3 to 4 feet long and uses a giant damselfly naiad's statistics, except it is slower (30 ft., swim 10 ft.) and gains the following action option.
 Jet Dash (Recharge 5–6). The dragonfly nymph swims up to 80 feet.

VARIANT: SMALL GIANT TENEBROUS & UNDERDARK DAMSELFLY NAIADS
These are larvae of monstrous damselflies adapted to live in dark environments (see Mammoth Helicopter Damselfly for details, under Tenebrous Helicopter Damselflies and Underdark Helicopter Damselflies). A small giant tenebrous damselfly naiad has blindsight 20 ft. and darkvision 60 ft.

A small giant underdark damselfly naiad uses a giant damselfly naiad's statistics, except it except it moves as well on land as water (30 ft., climb 30 ft., swim 30 ft.), has blindsight 40 ft. and darkvision 90 ft., and gains the following additional trait.
 Amphibious. The underdark damselfly naiad can breathe air and water.

VARIANT: SMALL GIANT TERRESTRIAL DAMSELFLY & DRAGONFLY LARVAE
This is a giant damselfly naiad or small giant dragonfly nymph adapted to live on land, although It cannot survive dry conditions for long (see Giant Dragonfly Nymph for details, under Terrestrial Giant Odonatid Larvae). A small giant terrestrial odonatid larva uses a giant damselfly naiad's statistics, except it moves better on land than water (30 ft., climb 20 ft., swim 10 ft.), loses the Jet Dash action option if it's a dragonfly nymph, and gains the following additional trait.
 Limited Amphibiousness. The terrestrial odonatid larva can breathe air and water, but it needs to be drenched in water at least once every 4 hours to avoid suffocating.

(Monster designed by Casimir Liber and Cleon on Enworld.org's General Monster Talk Creature Catalog Forum; inspired by the giant dragonfly nymph that debuted in EX2 The Land Beyond the Magic Mirror (1983) by Gary Gygax.)

 

[Casimir Liber]

Incidentally, in BECMI a dragonfly is literally a Dragon Fly. They're magical hybrids of chromatic dragons and anisoptera that look like giant-sized dragonflies with a somewhat draconic head.

Yes, obviously they have Breath Weapons, and they can use them AND bite every combat round.

Shall we do those too?

Oh god yes I remember these from BECMI! We have to do these first. Will have a look...

 

[Cleon]

Oh god yes I remember these from BECMI! We have to do these first. Will have a look...

I'd much rather do the "ordinary" Giant Dragonflies first, if only to have a CR 3 arthropod to add to the Exalted Thrall of Pazuzu's giant insect options.

After further mulling, I now fancy doing three sizes of giant Odonata.

The original AD&D version was Man-Sized, but came in two strength: regular 7 HD ones and tropical 8+1-8 HD ones.

So I'm thinking we could have "ordinary" Medium-sized Giant Dragonflies and Damselflies for the former and bigger Large-sized Giant Dragonflies for the latter. I'd be inclined to call the Large ones Emperor Dragonflies since big dragonflies can live in temperate climes, not just the tropics.

As for the Helicopter Damselfly, those specialize in eating spiders by plucking them out of their webs. A Giant Spider is a Large monster in 5E (indeed, I think they're Large in all editions of D&D), so a Giant Tropical Damselfly would have to be Huge to believably carry one off through the air.

Will update my placeholder entries to match.

 

[Cleon]

Odonatid, Giant (Imperial Giant Dragonfly)
Large beast, unaligned
Armor Class 15 (natural armor)
Hit Points 45 (7d8 + 7)
Speed 10 ft., fly 60 ft. (hover)

STR​	DEX​	CON​	INT​	WIS​	CHA​
18 (+4)​	18 (+4)​	13 (+1)​	1 (–5)​	12 (+1)​	5 (–2)​

Saving Throws DEX +6
Skills Perception +3
Senses passive Perception 13
Languages —
Challenge 3 (700 XP) Proficiency Bonus +2

Evasive Flight. A giant odonatid gains advantage on Dexterity saving throws when flying and attackers have disadvantage on attack rolls against it.
 If the giant odonatid is mounted or grappling an opponent, it must make a DC 12 STR check to use Evasive Flight each time it is attacked.

Expert Aerialist. The giant odonatid has advantage on Dexterity (Acrobatics) checks when flying.

Flyby. The giant odonatid doesn't provoke opportunity attacks when it flies out of an enemy's reach.

Keen Sight. The giant odonatid has advantage on Wisdom (Perception) checks that rely on sight.

Motion Camouflage. If the giant odonatid moves at least 20 feet towards a creature and then hits it with an attack on the same turn, that target must make a Wisdom (Perception) check against a Dexterity (Acrobatics) check by the odonatid. On a failure, the target cannot use Dodging Flight, Evasive Flight, or a similar defensive ability to avoid attacks by the giant odonatid until the start of its next turn.

Actions

Multiattack. The giant odonatid makes two attacks against a Medium or smaller creature. The first attack is a legs attack. If this succeeds in grappling the target, the odonatid makes a bite attack, otherwise it makes a second legs attack.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Legs. Melee Weapon Attack: +6 to hit, reach 5 ft., one Large or smaller creature. Hit: The creature is grappled (escape DC 14). Until this grapple ends, the target is restrained, and the giant odonatid can't make legs attacks against another target.

Bonus Actions

Dashing Flight. The giant odonatid flies up to 80 feet. It cannot use Burst of Speed if it used Dodging Flight after the start of its previous turn.

Reactions

Dodging Flight. If a creature of at least Small size attempts a melee attack against a giant odonatid, or the odonatid is targeted by a ranged weapon of similar size such as a giant-hurled boulder, the insect can try to dodge out of the way as a reaction. The giant odonatid makes a Dexterity saving throw with advantage (it loses this advantage when mounted or grappling an opponent). If the saving throw beats the attack roll or DC of the attack the giant odonatid takes no damage and flies up to 30 feet.
 If a giant odonatid is carrying or grappling a creature that is too heavy for the odonatid to fly with, it must release the creature to use Dodging Flight. If the odonatid is grappling a creature light enough to carry through the air, the grappled creature can make an escape check as a free action. If it fails, the odonatid may fly away with its victim.


Description

Apart from its size, an imperial giant dragonfly is no different from a standard giant dragonfly (see Giant Dragonfly for details). These insects are about 12 feet long and 1,000 pounds, with a 20 foot wingspan. Many species inhabit hot climates (these are called imperial giant tropical dragonflies), but quite a few live in temperate environments.
 Lions of the Sky. Imperial giant dragonflies are aggressive aerial predators who preferentially target Small and Medium flying creatures, particularly monstrous insects such as giant grasshoppers, giant wasps, and smaller giant odonates. Their fondness for eating regular giant damselflies and giant dragonflies probably explains why those are rare around the tropical rivers and lakes where imperials thrive. Tropical giant dragonflies tend to be extremely colorful, and the swirling brilliance of a flight of these odonatids earned them the collective name "a kaleidoscope of dragonflies."

VARIANT: IMPERIAL GIANT TROPICAL DAMSELFLY
While a standard giant damselfly is a Small beast, some tropical species grow much bigger (see Giant Damselfly for details). An imperial giant damselfly averages 15 to 18 feet long, with an 18 to 25 foot wingspan. It uses an imperial giant dragonfly's statistics except for being slower (fly 50 ft., Dashing Flight 40 ft., Dodging Flight 30 ft.). Their naiads are as dangerous as an imperial dragonfly nymph (see Imperial Giant Dragonfly Nymph for details).

VARIANT: IMPERIAL GIANT TENEBROUS & UNDERDARK HELICOPTER DAMSELFLIES
The majority of monstrous helicopter damselflies can see in the dark far better than any natural damselfly, with tenebrous species that hunt in the gloom of forests and nighttime and even subterranean varieties native to the underdark (see Mammoth Helicopter Damselfly for details, under Tenebrous Helicopter Damselflies and Underdark Helicopter Damselflies). An imperial giant damselfly of these variants uses an imperial giant dragonfly's statistics except for having better senses—the tenebrous variety has blindsight 30 ft., darkvision 90 ft.; the underdark variety, dubbed a wraithwing, has blindsight 60 ft., darkvision 120 ft.

(Monster designed by Casimir Liber and Cleon on Enworld.org's General Monster Talk Creature Catalog Forum; based on the tropical giant dragonfly that first appeared in EX2 The Land Beyond the Magic Mirror (1983) by Gary Gygax.)