5E: Monstrous Arthropods for Fifth Edition

Casimir Liber said:

They've all got such huge multifaceted eyes....so yes, I reckon perception proficiency is a must!

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I'll slap Perception on the next Updates.

Casimir Liber said:

Re cool varieties - am keen on the BECMI chromatic "dragon" flies -espeically as I have Tiamat as a villain warping critters!

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Just to be clear, by "cool variants" I was talking about Variants of the Giant Odonatids, as in "Variant: Some-Cool-Thing Damselfly" or "Variant: I-Wanna-Kill-Your-PC Nymph."

I wasn't planning on doing a "Variant: Draconic Odonatic" subentry just adds a Breath Weapon to a Giant Dragonfly.

The chromatic Dragon Flies make more sense as new, separate monsters.

I'm presuming the 5E versions will be small monstrosities rather than beasts likes the giant odonatids I've statted up. Both the Mystaran Monstrous Compendium (AD&D) and DMR2 Creature Catalog (BECMI) say the insect-dragon Dragonfly is Small, and being magical crossbreeds they'd be monstrosities in 5E like a Chimera, the archetypical unnatural hybrid.

ah ok. many years since L glanced at them

A word on scale.

My general rule-of thumb for Giant Arthropods is to scale them up by ×72 or ×100. The former turns an inch-long bug int a 6-foot bug, while with the latter a centimetre-long spider becomes a metre-long spider.

However, I might warp the scaling up or down a bit (e.g. ×36, ×50, ×144, ×180) if the resulting arthropod seems too big or small for its role.

For example, a Schizomid is "generally less than 5 millimetres (0.20 in) in length" (Wikipedia), which'd be 36 to 50 cm long with my usual scaling, about 14 to 20 inches. That'd be a Tiny beast in 5E and they need to be Small to eat adventurers, so I used a 180 scale and made the Giant Schizomid "rarely grow more than 3 feet long."

For weights, if I assume the density stays the same as the arthropod is "giantized" then its mass will increase by the cube of the linear increase. Thus, a ×100 scale insect becomes a million times heavier (100³), while a ×72 one is 373,248 times heavier (72³). The former is nice and neat. An insect that weighs 1 gram at its normal size would way 1,000 kg if enbiggened to a hundred times its size.

In the case of the Mammoth Helicopter Damselfly, according to the entomologist Ola Fincke*, on average you get about one giant damselfly per litre of water in the breeding site. If we scale that up by 100³ that's a million litres of water, equivalent in volume to a cube 10m square (which is around 32 feet 9½ inches each side, or 32,245 cubic feet). That's a million metric tons of water, by the way.

* see Use of forest and tree species, and dispersal by giant damselflies (Pseudostigmatidae) [Fincke, 2006]. Yes, I have been looking through research papers to write up my D&D Monster conversions. Everyone does that, right?

Then I though "hold on, ten metres is pretty close to the 30 foot length of the adult damselfly, so why not round it down to a 30-foot cube, for 27,000 cubic feet of water. That's a more convenient number to work with.

That's the basis for the water-pit volumes I used in the Description rough draft:

Cleon said:

 Water Pit Breeders. Natural helicopter damselflies breed in phytotelmata—the pockets of water that collect in vegetation (e.g. holes in trees, hollow tree stumps, broken bamboo, cup-shaped plants like bromeliads, fallen leaves and rinds). Males compete for the best breeding spots and guard the phytotelmata, mating with females before they lay their eggs. While normal helicopter damsels might have difficulty finding a phytotelma, it is almost impossible for mammoth helicopters, since only implausibly colossal plants could hold enough water. Giant helicopter damselflies only breed in still water, so rivers and streams are not an option. Instead, the giant helicopter damselflies usually breed in water-filled pits, such as sinkholes and caves, including artificial ones like cisterns and water tanks. A mammoth helicopter damselfly breeding site will, on average, produce a single adult for every 27,000 cubic feet of water. An Olympic-sized swimming pool (~88,000 cubic feet), for example, would produce around three adults, although as little as 12,000 cubic feet might be enough for a single naiad to reach full size (see Mammoth Damselfly Naiad for statistics).

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However, all of the giant odonatids I've statted up have used scalings smaller than ×100, so that's WAY too big.

I've been sliding the scaling about a bit to get "typical specimen" a Medium monster to match the size of the original AD&D monster.

An average dragonfly is maybe 2 inches long (51mm) while a big one like an emperor dragonfly is around 3 inches. If those are matched to the 7½ foot Giant Dragonfly and 12 foot Imperial Giant Dragonfly that's a ratio of ×48.

However, a 5-inch (127mm) giant helicopter damselfly becoming a 30-foot Mammoth Helicopter Damselfly is a ×72 scaling, so I should use that to calculate the water pit productivity. So instead of a million litres, I should use 373,248 litres. A cubic foot is 28.31685 m³, so that's 13,181 cubic feet. Let's just call it 13,500 feet and halve the current volumes:
So if the minimum water pit size is 6,000 cubic feet, we're talking a water tank 10 feet deep by feet by 20 feet wide by 30 feet long; or a 12-foot diameter sinkhole with water 53 feet deep; or a 39-foot diameter pond that averages 5 feet deep.

Hmm, I wonder how big the largest Meganisoptera would become if scaled up at those ratios?

Estimates of Meganeuropsis and Meganeura appear to go up to 35cm length, 75 cm wingspan, and 150g weight. That's roughly 14 inches long, 30 inch wingspan, and about a third of a pound.

If I use the mammoth helicopter damselfly scale of ×72 that comes to…

…82½ feet long, 177 foot wingspan, 123,320 pounds or just under 62 short tons.

Yowza! That's as heavy as a sperm whale!

Contrariwise, if I use the same ×48 scaling as the Giant Dragonfly it comes to 55 feet long, 118 foot wingspan, 36,539 pounds or 18 tons, which is not that much higher than the current guesstimate I've got for the Titanic Tropical Dragonfly I'm thinking of statting up.

Titanic Tropical Dragonfly sounds really fun... Giant Maganeura....

Casimir Liber said:

Titanic Tropical Dragonfly sounds really fun... Giant Maganeura....

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From what I can tell, Meganeuropsis is thought to be even bigger than a Meganeura. It's hard to tell, since the fossils of the latter are even fewer and more fragmentary than Meganeuropsis remains.

I reckon the Meganisoptera are likely more analogous to the Giant Damselflies I've been statting. The modern reconstructions look a lot lighter and spindlier in build than a Dragonfly.

Anyhow, better get back to my Mammoth Helicopter Damselfly's descriptions…

Who knows, I might even get it done today!

Here's the "juicy bit" of the Mammoth Helicopter.

I intend to write up a Description with all sorts of interesting fluff for the Underdark variant, but the following contains all the rule elements.
My first attempt used the CHA-based save DC standard to enchantment type effects, but that was only DC 9 which seemed way too low so I made it DEX-based on the basis that it's a motion-based ability. Thought about a WIS-based DC 12, but that didn't make much sense to me, and having the CHA-based DC 9 with disadvantage if being viewed with darkvision, but to do that I'd have to make the phrasing more complicated and it didn't seem worth it.

Since I've made decent progress on the Mammoth helicopter and come up with some more tweaks to the stats & descriptions of the others, it's time for another update.

Updating the Giant Damselfly.
Updating the Giant Dragonfly.
Updating the Giant Emperor Dragonfly.
Updating the Giant Tropical Damselfly.

EDIT: Better post placeholders for the other sizes I'm currently thinking about! :ENDEDIT

Added Odonatid, Giant (Titanic Tropical Dragonfly).
Added Odonatid, Giant (Titanic Dragonfly Nymph).
Added Odonatid, Giant (Diminutive Giant Dragonfly).
Added Odonatid, Giant (Diminutive Giant Dragonfly Nymph).
Added Odonatid, Giant (Dainty Giant Damselfly).
Added Odonatid, Giant (Dainty Giant Damselfly Naiad).

Odonatid, Giant (Titanic Tropical Dragonfly)
Huge beast, unaligned
Armor Class 15 (natural armor)
Hit Points 95 (10d12 + 30)
Speed 10 ft., fly 80 ft. (hover)


Saving Throws DEX +6
Skills Perception +4
Senses passive Perception 14
Languages —
Challenge 7 (2,900 XP) Proficiency Bonus +3

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 Large 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: +10 to hit, reach 5 ft., one target. Hit: 25 (4d8 + 7) slashing damage.

Legs. Melee Weapon Attack: +10 to hit, reach 10 ft., one Huge or smaller creature. Hit: 10 (1d6 + 7) bludgeoning damage and the creature is grappled (escape DC 18). 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 40 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

These awesome aerial predators may be shorter than a mammoth helicopter damselfly but are even deadlier, since they are as aerobatic and aggressive as smaller giant dragonflies (see Giant Dragonfly for details). Dragonflies of this monstrous size are very rare and usually only found in hot environments with plenty of water and prey. A titanic dragonfly is 24 to 27 feet long, with a 40 to 45 foot wingspan, and a weight of 10,000 pounds or so.

VARIANT: TITANIC TROPICAL DAMSELFLY
The mammoth helicopter damselfly is the largest damselfly known to exist on the surface world (see Mammoth Helicopter Damselfly for details), but the underdark giant damselfly (see below) sometimes exceeds even its enormous size, and tropical damselflies that are "bigger than mammoth" are rumored to exist in the menagerie of an eccentric mage who has perfected the <i>giant insect</i> spell. A titanic tropical damselfly would average 40 feet long, with an 40 to 60 foot wingspan. It uses a titanic tropical dragonfly's statistics except for being slower (fly 60 ft., Dashing Flight 40 ft., Dodging Flight 40 ft.). Its naiads are just as lethal as titanic dragonfly nymphs, if arguably less horrible in appearance (see Titanic Dragonfly Nymph for details).
 A titanic damselfly's statistics can also be used for a monstrous giant griffinfly—the griffinflies or Meganisoptera are prehistoric ancestors of the odonatids, and the biggest natural griffinfly far exceeded any normal odonatid in size, being a match for the smallest giant damselflies (see Dainty Giant Damselfly for statistics).

VARIANT: TITANIC 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 titanic tenebrous damselfly has blindsight 30 ft. and darkvision 90 ft.

A titanic underdark damselfly, commonly dubbed a death phantom, has a challenge rating of 8 (3,900 XP) and uses a titanic tropical dragonfly's statistics, except it has blindsight 60 ft., darkvision 120 ft., and gains the following additional trait and action option.
 Magic Resistance. The death phantom damselfly has advantage on saving throws against spells and other magical effects.
 Hypnotic Wings. The death phantom damselfly swirls its wings in a hypnotic pattern. Every creature within 300 feet of the damselfly that can see the wings and is not another underdark damselfly must succeed on a DC 14 Wisdom saving throw or be charmed until the wings' motion ends. If a creature has Fey Ancestry and sees the Hypnotic Wings with darkvision, the trait does not give the creature advantage on saving throws against being charmed by the wings.
 The death phantom damselfly must take a bonus action on its subsequent turns to continue the hypnotic motion. It can stop its hypnotism at any time. The hypnotism ends if the death phantom is incapacitated or flies more than 60 feet on its turn (including any distance it moves with Dashing Flight or Dodging Flight). While charmed by the death phantom damselfly, a creature is incapacitated and does nothing but stare mesmerized at the insect. The hypnosis effect ends if the target takes any damage from the death phantom damselfly. If a charmed creature takes damage from a source other than the death phantom or is about to face another obvious danger, such as an approaching fire, it can repeat the saving throw. A charmed creature can also repeat the saving throw at the end of each of its turns. If the saving throw is successful, the effect ends on it. A creature that successfully saves is able to resist the hypnotism for the next 24 hours. If the creature fails a saving throw during their period of resistance against the death phantom damselfly's Hypnotic Wings, it is only partially charmed and may target the death phantom with harmful abilities or magical effects. Instead of being incapacitated, a partially charmed creature has disadvantage on attack rolls and skill checks, and the death phantom damselfly has advantage on attack rolls against the partially charmed creature.

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

Odonatid, Giant (Titanic Tropical Dragonfly Nymph)
Huge beast, unaligned
Armor Class 15 (natural armor)
Hit Points 105 (10d12 + 40)
Speed 30 ft., swim 10 ft.


Skills Stealth +5
Senses blindsight 30 ft., passive Perception 11
Languages —
Challenge 6 (2,300 XP) Proficiency Bonus +3

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: +10 to hit, reach 5 ft., one target. Hit: 28 (6d6 + 7) slashing damage and the target is grappled (escape DC 18). 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: +10 to hit, reach 10 ft., one target. Hit: 28 (6d6 + 7) piercing damage. If the target is a Gargantuan or smaller creature, the target is grappled (escape DC 18) and must make a DC 18 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 120 feet.


Description

Apart from their size, these larvae are no different from a standard giant dragonfly's larva (see Giant Dragonfly Nymph for details). A titanic tropical giant dragonfly nymph grows about 18 feet long.

VARIANT: TITANIC TROPICAL DAMSELFLY NAIAD
This is the larva of a Huge giant damselfly of exceptional size (see Titanic Tropical Dragonfly for details), even bigger than a mammoth damselfly naiad. A titanic tropical damselfly naiad is roughly 25 feet in length and uses a titanic tropical dragonfly nymph's statistics, except it is faster (30 ft., swim 30 ft.) and loses the nymph's Jet Dash action option.

VARIANT: TITANIC 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 titanic tenebrous damselfly naiad has blindsight 30 ft. and darkvision 90 ft.

A titanic underdark damselfly naiad, also known as death phantom naiad, has a challenge rating of 7 (2,900 XP) and uses a mammoth damselfly naiad's statistics, except it moves as well on land as water (30 ft., climb 30 ft., swim 30 ft.), has blindsight 60 ft. and darkvision 120 ft., and gains the following additional traits and action option.
 Amphibious. The death phantom naiad can breathe air and water.
 Magic Resistance. The death phantom has advantage on saving throws against spells and other magical effects.
 Shimmer of Imperceptibility. The death phantom naiad vibrates its wing-buds to create a strange shimmering. Any creature that sees it must succeed on a DC 13 Wisdom saving throw or be charmed until the wing-buds' vibration ends. If a creature has Fey Ancestry and sees the Shimmer of Imperceptibility with darkvision, the trait does not give the creature advantage on saving throws against being charmed by the shimmer. The death phantom naiad must take a bonus action on its subsequent turns to continue the shimmering. It can stop the shimmering at any time. The shimmering ends if the death phantom naiad is incapacitated or moves more than 10 feet on its turn.
 While charmed by the shimmer, a creature becomes unable to perceive the death phantom naiad with any form of sight apart from truesight, so the naiad becomes invisible to the creature, even to blindsight and the see invisibility spell. Unlike a normal charm, the charmed creature can still attack the death phantom or target the naiad with harmful abilities or magical effects, and the naiad does not gain advantage on social interaction ability checks with the creature.
 The hypnosis effect ends if the target takes any damage from the death phantom naiad. If a charmed creature takes damage from a source other than the death phantom or is about to face another obvious danger, such as an approaching fire, it can repeat the saving throw. A charmed creature can also repeat the saving throw if it takes an action to try to see the death phantom naiad, but if the naiad is trying to hide the creature must first succeed at a Perception check against the naiad's Stealth check to be allowed the attempt. If the saving throw is successful, the effect ends on it. A creature that successfully saves is able to resist the shimmer for the next 24 hours. If the creature fails a saving throw during their period of resistance against the death phantom naiad's Shimmer of Imperceptibility, it is only partially charmed and merely has trouble seeing the insect, with the same effect as the blur spell (the partially charmed creature has disadvantage to attack the naiad, the naiad does not gain advantage on its attacks).

VARIANT: TITANIC TERRESTRIAL DAMSELFLY & DRAGONFLY LARVAE
This is a titanic damselfly naiad or titanic 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 titanic terrestrial odonatid larva uses a titanic damselfly 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.

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