Morphology

The immature stages of black flies are adapted for aquatic life, although the nonmobile pupa also has terrestrial adaptations that are useful if the water recedes. The egg is roughly oval or triangular, with rounded angles. It has a glutinous outer layer and a smooth, pigmented inner shell. A micropyle, consisting of a simple hote in the egg for the entry of sperm, is present in some species but not others.

FIGURE 11.3 Pupae of the North American black fly Simulium vit-tatum (copyright Dwight R. Kuhn).

FIGURE 11,2 Larva of the North American black fly Simtdmm lujjgeri (original by Lawrence W. Zettler).

The larva (Fig. 11.2) hatches with the aid of an egg burster, a small tubercle on the dorsum of the head capsule. The basic larva! design consists of a well-sclerotized head capsule bearing an anterior pair of labral fans, and an elongate body with one thoracic proleg and a terminal abdominal proleg. Rows of tiny hooks on the prolegs enmesh with silk pads spun from the pair of larval silk glands and applied to a substrate. These silk glands extend from the anterior of the head into the posterior portion of the abdomen where they enlarge and double back on themselves. The adhesiveness of the silk is correlated with the velocity of the flowing water to which each species is adapted.

While clinging to a pad by its posterior proleg, the larva extends its body to filter feed. The prominent labral lans, each with about 20—80 individual rays bearing mi-crotrichia (minute hairs) on their inner surface, are used to filter particulate matter from the water current. Larvae of some species (e.g., Gymnopais spp.) that live in habitats, such as glacial meltwaters, with little suspended food have lost the labral fans over evolutionary time. These species rely on their mandibles, specialized labrum, and hypostoma to scrape food from the substrate.

Additional features of the head and body are conspicuous and taxonomically important. The antennae, which consist of three articles and a terminal cone sensillum, are elongate, slender, and variously pigmented. A pair of dark eyespotsis prominent on each side of the head capsule. Pigmentation patterns of the bead capsule and body and the shape of the postgenal cleft, an area of weakly sclerotized cuticle on the ventral side of the head capsule, are important for interpreting the taxonomy of the family. The anteroventral portion of the head capsule bears the hypostoma, an anteriorly toothed plate used in conjunction with the mandibles to cut strands of silk and to scrape food from the substrate. Mature larvae are recognized by the presence of a prominent, dark gill histoblast on each side of the thorax.

The pupa (Fig. 11.3), which resembles an adult with its appendages held close to the body, is housed in a silk cocoon. Cocoons are shapeless sacs in the evolutionarily basal species but are well-formed, slipper- or boot-shaped coverings, sometimes bearing anterior processes and lateral windows, in the more derived species. The pupa is held firmly in its cocoon by numerous anteriorly directed sets of hooklets. A pair of conspicuous gills arises from the thorax. The gills are among the most taxonomically useful and fascinating structures in any life stage. They vary in arrangement from thick, clublike structures to clusters of 2 to more than 100 slender filaments.

FIGURE 11.3 Pupae of the North American black fly Simulium vit-tatum (copyright Dwight R. Kuhn).

(Copyright Roelof Idema.)

The adult black flies (Fig. 11.4) are characterized by a small but robust body, conical or beadlike antennae with seven to nine flagellomeres, and an arched thorax bearing a pair of wings that typically span 6—10 mm and have thickened veins near the leading margin. Most species are blackish, but orange, yellow, and variously patterned species also exist. Males of nearly all species have holoptic eyes that occupy most of the head and meet at the midline. Male eyes consist of enlarged dorsal facets, in addition to the typical-sized ventral facets, an arrangement that enhances the ability of males to locate females entering a mating swarm from overhead. Females have smaller, di-choptic ms separated by the frons (Fig. 11.4).

The mouthparts arise ventrally from the head. A conspicuous pair of long maxillary palps attaches near the base of the proboscis. The third segment accommodates the sensory vesicle (Lutz's organ), which has many chemosen-silla that detect odors such as carbon dioxide. The labium forms the back of the proboscis and envelops the other mouthparts, including the minutely serrated mandibles and the toothed laciniae, with a pair of large, fleshy lobes called the labella. The mouthparts of the male are similar to those of the female, except the mandibles and laciniae are not adapted for blood feeding and, therefore, do not bear teeth.

The stout thorax bears a pair of wings, either smoky or hyaline but never patterned. The venation, including the setation, is taxonomically important at the generic level. The color patterns of the legs and thoracic scutum are useful for species identification. The tarsal claws exist in one of three conditions. Species that feed on mammals have either a simple, unarmed claw or a minute tooth at the base of each claw. Bird feeders are endowed with a large thumblike lobe at the base of each claw. The abdomen is weakly sclerotized except the genitalia, which are of the utmost importance in the identification of specks. To interpret the taxonomically important characteristics of the genitalia of both males and females, the abdomens must be treated with a clearing agent such as potassium hydroxide or hot lactic acid and slide mounted in glycerin.

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