The Oestridae family of flies, commonly known as botflies, is distinguished by a parasitic larval stage that develops inside a host mammal. This obligate parasitic behavior means the larvae must live within a host to complete their development, leading to the condition known as myiasis. The adult botfly is a short-lived insect focused entirely on reproduction, while the larva is a specialized, feeding organism.
The Adult Form: Appearance and Mimicry
The adult botfly is a relatively large, robust insect that often confuses observers because it closely resembles a bumblebee. This visual deception is a form of Batesian mimicry, where a harmless species imitates the warning signs of a well-defended species to deter predators. Adult botflies are typically chunky and hairy, contributing to their fuzzy appearance.
The size varies by species; for example, the human botfly, Dermatobia hominis, measures approximately 12 to 19 millimeters long. While some species exhibit black and yellow bands like a bee, others may display a mix of colors, such as a yellow face, a dull blue thorax, and a metallic blue abdomen.
A key physical identifier distinguishing the adult botfly from its stinging look-alikes is the condition of its mouthparts. Adult botflies possess vestigial, or non-functional, mouthparts, meaning they are incapable of feeding. They must rely on energy reserves accumulated during the larval stage, resulting in a very short lifespan dedicated solely to mating and egg-laying.
Identifying the Larval Form
The botfly larva is the stage most associated with the insect and is adapted for its role as an internal parasite. These larvae are generally pale, creamy-white, or yellowish and have a segmented, grub-like body that can grow up to 25 millimeters in length. The body is typically barrel-shaped or cylindrical in its final developmental stage.
The most defining visual feature is the presence of numerous rows of specialized, backward-projecting spines or bristles encircling the body segments. These spines help the larva anchor itself securely within the host’s tissue and prevent easy removal. As the larva grows, it creates a boil-like swelling, or warble, in the host’s skin, maintaining a small external opening to the air.
At the posterior end of the larva, a pair of dark, sclerotized structures called spiracles are visible. These respiratory openings are positioned to protrude from the host’s breathing pore, allowing the larva to access oxygen while remaining embedded. The specific shape and arrangement of these posterior spiracles are important visual details for species identification. For example, the third-stage human botfly larva is cylindrical with spines in distinct rows, while earlier stages may appear more pyriform or bottle-necked.
Distinctions Between Major Types
The visual appearance of botfly larvae provides important clues to their species, particularly concerning their internal habitat within the host. The most significant distinction is the ultimate location of the larva, which dictates its shape and spine arrangement.
Larvae of the New World genus Cuterebra and the human botfly, Dermatobia hominis, cause furuncular myiasis, where a single larva develops in a localized, boil-like lesion under the skin. In contrast, the larvae of Hypoderma species, commonly known as cattle grubs or warble flies, also develop subcutaneously, but their movements can sometimes cause a migratory track beneath the skin. Hypoderma larvae are often initially smaller than the final-stage Dermatobia larvae. The fully grown Hypoderma larva is robust, but the specific pattern and coloration of its spines differ from the dense, dark rows seen on the human botfly.
The third major group, the Gasterophilus species, or horse botflies, presents a difference because their larvae develop within the digestive tract of their primary hosts, not under the skin. These larvae are adapted to withstand the harsh environment of the host’s stomach or intestines, and their overall shape is less stout and more elongated than the skin-dwelling types.