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:
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).
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:
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. On average, these sites produce one adult for every 13,500 cubic feet of water they contain, and pits as small as 6,000 cubic feet can be enough for a single naiad to reach adulthood. An Olympic-sized swimming pool (~88,000 cubic feet), for example, could produce six or seven adults from its resident naiads (see Mammoth Damselfly Naiad for statistics).
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.
