Giant Black Water Scavenger Beetle
Hydrophilus acuminatus
An underwater scavenger with its own air tank
The giant black water scavenger beetle is one of the largest aquatic beetles, its glossy black back smooth and streamlined. It resembles the fierce diving beetle but behaves quite differently: adults mainly eat water plants and decaying matter, paddling their legs side to side like an unhurried stroll along the bottom.
Key figures
- Length
- About 32–40 mm, one of the largest aquatic beetles in China
- Life cycle
- One generation a year; adults are long-lived and can overwinter underwater
- Habitat
- Ponds, rice paddies, and slow-flowing still water
- Diet
- Adults mainly eat water plants and decaying debris; larvae are carnivorous and prey on snails
- Air storage
- A coat of fine hairs on the underside traps a layer of air that glints silver underwater
- Egg-laying
- Females spin a silk egg case anchored beneath floating plants, drifting with the current
Life cycle
Egg (silk egg case) → Larva (carnivorous, aquatic) → Pupa (soil chamber on shore) → Adult
Ecological role
Adults feed on water plants and debris, helping recycle organic matter in still water; the carnivorous larvae prey on pond snails and other aquatic snails, helping to hold down populations of intermediate hosts for some parasites — a link in the food web of paddy wetlands.
Did you know
Its maxillary palps are longer than its antennae, taking over the job of feeling the way ahead; the short, club-shaped antennae are dedicated to breathing, drawing air down to the layer on its belly.
Range
Still waters across most of China, Japan, the Korean Peninsula, and Russia's Far East
Status
Common, though populations have declined in areas where paddy wetlands have shrunk
Anatomy
- Maxillary Palp — A sensory feeler (maxillary palp) longer than the antennae, used to feel the way ahead and sniff out food
- Keel — A sharp ridge on the mid-thorax underside that can jab a fish in the mouth if it attacks
- Elytra — Glossy black and streamlined, the wing cases (elytra) cut water resistance while diving
- Compound Eye — Set on either side of the head, taking in both the world above and below the water
- Swimming Leg — The mid and hind legs are fringed with swimming hairs, paddling side to side like an underwater stroll
- Antenna — Short and club-shaped, it breaks the surface to draw fresh air down into the air layer on the belly
Open the interactive 3D specimen