On the Life History and the Anatomy of the Early Stages of Forcipomyia (Diptera, Nemat., Ceratopogoninae)

Abstract

(1) Forcipomyia is one of the few genera of the Ceratopogoninae (Chironomidae) whose larvae are not aquatic. They retain, however, the apneustic condition typical of the entire family, and are therefore compelled to live in moist places such as beneath the rotting bark of fallen trees, in the hollow tap roots of dead plants, in wounds and rot holes in trees, in drying horse and cow manure, in decaying fungi, and in the nests of ants. The immature forms are strongly gregarious.(2) The larvae pass through four stages, each lasting about a week; the successive changes in chaetotaxy at each moult is recorded in the discussion of F. picea. All known European species of this genus hibernate as third or fourth stage larvae. The duration of the pupal stage is about one week; the length of life of the adult has not been determined. There are usually three generations per annum. The adults are known to suck the blood of other insects.(3) The larvae are eucephalous, more or less cylindrical, 3–6 mm. long, with eleven clearly denned segments (three thoracic and eight abdominal, the eighth divided into two apparent segments). Progression is achieved by means of a retractable, biramous, prothoracic pseudopod, and a ventral hook-studded ridge on the ultimate abdominal segment, with the occasional aid of the mandibles.(4) The head and body bear numerous lanceolate and simple hairs of great specific importance.(5) The mouth-parts consist of a fleshy labrum, a pair of small premandibles, a pair of long, slender mandibles working in the sagittal plane on either side of the labrum, a pair of reduced maxillae of fleshy nature with chitinous internal supports, and a simple untoothed labium. Within the head there is a complicated chitinous structure, the hypopharynx; this is figured and described in full detail and its action in comminuting the food is revealed for the first time.(6) No investigation has hitherto been made of the internal anatomy. The alimentary canal, consisting of buccal cavity, hypopharynx, oesophagus, mid-gut, and hind-gut, is quite simple in structure, without nodules or caecal chambers. It is only slightly longer than the body, a loop occurring in the hind-gut when the larva contracts.(7) The malpighian tubules are either two or three in number, the latter condition being unique among insects. The third is ventral, passing into the posterior part of the body only.(8) The salivary glands are a pair of S-shaped bodies closely adhering to the walls of the anterior portion of the mid-gut; they are chiefly composed of large binucleate cells.(9) The circulatory system consists of a dorsal vessel extending throughout the body and passing between the lobes of the brain above the oesophagus into the head. In the eighth abdominal segment it enlarges slightly to form the heart, with one pair of ostia or valves situated on its posterior extremity.(10) The nephrocytes are arranged segmentally, four to each abdominal segment and a single mass in the thorax, all attached to the dorsal vessel. The excretory function of these cells was demonstrated by injection of ammonia-carmine into the body cavity.(11) The nervous system is only very slightly cephalised, there being seven ganglia for the eight abdominal segments and three in the thorax; the normal bilobed brain is situated in the thorax in all larval stages. The sympathetic system is described, and differs somewhat from that of its close relative, Chironomus.(12) Despite the absence of spiracles there is a well-developed tracheal system in the form of two main trunks with transverse connections, branch tracheae, and subcutaneous plexuses.(13) The muscular system, oenocytes, fat-body, and gonads have also been studied in detail.(14) The larval skin is retained upon the last four or five abdominal segments of the pupa.(15) The characters used for determining the species, in their order of importance, are as follows: Larva: chaetotaxy, prothoracic pseudopod, markings of the head, anal blood-gills. Pupa: cuticular spines, prothoracic horns.(16) The larvae and pupae of nine species of Forcipomyia are described with particular reference to their specific characters. Descriptions of the adults of two new species, F. radidcola and F. pulchrithorax, by Mr F. W. Edwards, are appended.(17) Closely related forms are Euforcipomyia and other exotic subgenera not yet named, and the genus Atrichopogon (now including Kempia). Dasyhelea belongs to this group of hairy-winged flies, but its larvae partly resemble those of the bare-winged Ceratopogonines in that they have lost the pro-thoracic pseudopod and macrochaetae, but retain the anal hooks.