The caddis fly, an insect belonging to the order Trichoptera, is often mistaken for a small moth. Its scientific name translates to “hair-winged,” describing its most distinguishing adult feature. Caddis flies are closely related to moths and butterflies, but their early stages are entirely aquatic. These insects are most often encountered near clean rivers, lakes, and streams, which serve as habitats for their larval forms.
The Distinctive Adult Appearance
Adult caddis flies are generally small and slender, typically ranging from a quarter of an inch to nearly an inch long, though some species reach 1.5 inches. They are usually colored in drab hues of brown, gray, or green, providing effective camouflage against bark or streamside vegetation. Their overall shape is delicate, with a tapered abdomen that gives them a fragile appearance.
The wings are a primary feature of the adult caddis fly. They possess two pairs of wings that, when at rest, are held in a steep, tent-like position over the body. This resting posture sets them apart from many other insects. Unlike the dusty scales found on moths, caddis fly wings are covered in fine, dense hairs, giving them a slightly fuzzy texture.
The antennae are long, thin, and thread-like, composed of many segments. In many species, the antennae can be as long as the entire body and are often held swept back over the wings or pointed forward. The head also features large, well-separated compound eyes. Adult caddis flies generally have reduced or non-functional mouthparts, as they are typically short-lived and focus on reproduction.
The Unique Larval Stage
The appearance of the caddis fly during its immature stage is unique. The aquatic larvae are grub-like, growing between a half-inch to one inch long. They have a segmented abdomen, three pairs of legs near the head, a distinct head capsule, and chewing mouthparts, unlike the vestigial mouthparts of the adults.
Most caddis fly larvae construct portable cases that serve as protective shelters. These cases are tubes spun from silk secreted from glands near the mouth, reinforced with materials gathered from the environment. Materials used can include tiny pebbles, sand grains, bits of leaves, or small twigs, all cemented together into a durable structure.
These cases vary widely in shape; some are cylindrical, others are conical, and a few resemble small snail shells. The larva remains inside the case, pulling it along the streambed, allowing only its head and three thoracic segments to protrude. Certain species, known as “free-living caddis flies,” do not build portable cases but instead construct fixed silken retreats or nets to capture food particles in the water.
How to Tell a Caddis Fly from a Moth
Telling a caddis fly from a moth requires looking at three distinct features. The primary difference is wing texture: a caddis fly’s wings are covered in hair, while a moth’s wings are covered in fine, easily detached scales that often come off as a dusty powder. The wings of a caddis fly are also often more translucent and membranous.
The way the insect holds its wings at rest provides another clear separation. Caddis flies fold their two pairs of wings steeply over their body in a characteristic tent shape. While some moths rest this way, many moth species hold their wings flatter over their back or display them open.
Finally, a look at the mouthparts can confirm the identity. Many adult moths possess a long, coiled proboscis used to feed on nectar or other liquids. Adult caddis flies generally lack this structure, possessing only reduced or vestigial mouthparts.