Identify Larvae: A Legs-and-Head Key to Insect Larvae

You can identify most insect larvae to their group by counting legs and checking for a head. The University of Kentucky's larval key starts with one question: does the larva have segmented legs on the thorax, the three segments right behind the head? Then:

• True legs plus prolegs on 5 or fewer abdominal segments: a caterpillar, the larva of a moth or butterfly (UC IPM). See caterpillar identification.
• Prolegs on 6 or more abdominal segments: a sawfly larva, which is a relative of wasps (UC IPM).
• True legs, no prolegs: a beetle larva, or a lacewing larva (UC IPM; University of Kentucky).
• No true legs, but a distinct head: weevil grubs, mosquito larvae (wigglers), midges, drain flies, fungus gnats and soldier flies (University of Kentucky).
• No legs and no visible head: maggots of flies, crane fly larvae and some wood borers (University of Kentucky).

Prolegs are the fleshy, unjointed stubs on the abdomen; true legs are the small jointed ones behind the head.

Among larvae with true legs, the University of Kentucky key separates two shapes:

• Long legs, streamlined and often pointed body, flat head with forward-pointing jaws: predators such as lacewing, lady beetle and ground beetle larvae.
• Short, thick legs, a box-shaped head and a wider abdomen: plant feeders such as white grubs, wireworms, rootworms and leaf beetle larvae. See grub identification.

Behaviour helps too. On the iNaturalist forum, entomologists point out that sawfly larvae often hold their bodies in an S shape, geometrid caterpillars loop like inchworms, and antlion larvae scoot backwards.

A maggot is one kind of larva: the larva of a fly. Every maggot is a larva, but most larvae are not maggots. UC IPM separates fly larvae from all the others because they lack true legs and have fleshy bumps on the abdomen or thorax instead, and many have no visible head (University of Kentucky). A caterpillar, grub or wireworm has jointed legs; a maggot does not.

Larvae that look like maggots but are not house fly maggots include rat-tailed maggots, the larvae of drone flies, which live in stagnant water and have a breathing tube at the rear (Wikipedia, Hoverfly), and the headless aphid-eating maggots of hover flies on garden plants (University of Kentucky). Weevil grubs are legless too, but have a distinct head. See maggot identification.

Indoors, where you find the larva usually tells you what it is. Sizes are from the sources named.

• In wool, carpets or under furniture, with silk tubes or a small case: clothes moth larvae, shiny white with a dark head (UC IPM), up to about 13 mm (Natural History Museum, London). Damage hides under collars and cuffs and in carpet beneath furniture.
• Hairy, often tapering at one end, leaving brown bristly cast skins and no webbing: carpet beetle larvae (UC IPM). Black carpet beetle larvae can be 5/16 inch long, tapering to a tuft of long hairs.
• In flour, cereal or nuts, with silk threads everywhere: Indianmeal moth larvae, about 1/2 inch with a greenish or pinkish tint (UC IPM).
• In stored grain products, short and bristly: warehouse beetle larvae, about 1/4 inch with stiff hairs from dark plates (UC IPM).
• In flour, pale and slender: flour beetle larvae, about 3/16 inch, white tinged with yellow (UC IPM).
• Near pet food, dried meat or dead insects in an attic: larder beetle larvae, about 1/2 inch, hairy, with two curved spines at the tail (Penn State Extension).
• In a sink or shower drain: often drain fly larvae, legless but with a distinct head (University of Kentucky key).
• White, legless and headless near garbage: fly maggots.

For the adults that go with these, see household bug identification.

Usually to the group, often to the species for common pests, but not always further. Larvae change shape as they grow, and experienced entomologists on the iNaturalist forum note that precise identification can need a microscope and a technical key. The Prairie Haven moth caterpillar gallery, with more than 200 species, confirms most of its identifications by rearing the larva to an adult.

To give a photo the best chance:

• a side view that shows the legs along the whole body, so prolegs can be counted;
• a close view of the head;
• something for scale, such as a coin or ruler;
• a note of where you found it and what it was eating.

InsectLens names larvae from a photo; if the result is only a group, the leg count above will usually tell you whether it matters.

Research and drafting for this page used AI assistance. Every figure, date and quotation was then checked line by line against the sources listed at the bottom of the page, each of which was opened while writing. Nothing here is a diagnosis. If a bite, sting or contact with an insect is followed by trouble breathing, swelling of the face or throat, spreading redness, fever or any symptom that worries you, get medical care; in the United States Poison Help (1-800-222-1222) is free and open 24 hours a day.

University of Maryland Extension's household pest guide lists these larvae as the ones most often found indoors, all rated as nuisance pests:

• Fly maggots (house or blow flies): 1/2 inch or slightly longer, whitish, no visible head; near trash, on floors, in fireplaces and vents.
• Indianmeal moth larvae: 1/2 inch, whitish with a brown head; in stored products or crawling on walls.
• Carpet beetle larvae: 1/4 inch or less, carrot-shaped and furry; in closets, bathrooms and kitchens. See carpet beetle identification.
• Flea larvae: about 1/5 inch when mature, worm-like and whitish; in carpet, cracks in wood floors and pet bedding. See fleas.

Two more worth knowing: clothes moth larvae, shiny white with a dark head, in wool (see clothes moth identification), and mealworms, the tan-to-brown larvae of the darkling beetle Tenebrio molitor, up to about 1.25 inches (Wikipedia).