LICE AND FLIES. 



603 



throughout the whole of the Order, the hind-wings being represented only by 

 two knobbed appendages like drumsticks, which are known as 

 halteres or poisers. They are furnished with a proboscis, General 

 with which they imbibe their food, which consists, in the Characteristics, 

 perfect state, of liquid substances only. They undergo a 

 complete metamorphosis, their larvae being footless maggots, which pass the 

 pupa state in a barrel-shaped integument formed of the dried skin of the 

 larva. In the larval state, many species live in water, some in the ground, 

 others on plants, sometimes forming galls, while others feed on carrion, 

 dung, etc.; and others, again, are parasitic on various animals, neither 

 vertebrate nor invertebrate animals being secure from their attacks. They 

 probably form one of the largest orders of insects, but have been so little 

 studied that only 28,000 species were described up to 1892, of which between 

 2,000 and 3, 000 'inhabit the British Islands. The Flies are divided into two 

 principal sections, and the Aphaniptera, or fleas, are now usually regarded as 

 forming a third main section of Diptera rather than a separate order. 



The body is not generally very hairy, and the wings are usually destitute 

 of hair or scales, generally transparent, and with very few nervures, most of 

 which are longitudinal. The Aphaniptera have the barest rudiments of wings, 

 while some genera of Diptera are provided with small additional lobes at the 

 base of the wings, which are usually called alulae. 



DIPTERA NEMOCERA (GNATS AND CRANE-FLIES). 



The insects belonging to this group are generally slender-bodied flies, with 

 the antennae long, or of moderate length, and composed of six joints and 

 upwards. The palpi have from three to six joints. They frequently undergo 

 their metamorphoses in the water or in the ground ; and none of the species 

 of this section are parasitic, or even carnivorous, in the larval stage, though 

 the females of certain families will suck blood in their perfect state. 



The Cecidomyiidce, or gall-gnats, with which we will commence our notice 

 of the Nemocera, are very small, gnat-like insects, with long moniliform 

 or cylindrical antennae, often set 

 with whorls of hairs, and compara- - Family 

 tively broad, iridescent wings, with CecidomyiidcK. 

 from two to five longitudinal ner- Gall-Gnats, 

 vures only, and these frequently 

 not all clearly defined. Their bodies are clothed 

 with long hairs. The larvae are very various in 

 their habits, many of them causing galls, on various 

 trees and plants, especially willows, while others 

 live in bulbs, fungi, rotten wood, under bark, or in 

 fir-cones. One or two species are at times very 

 destructive to wheat, especially the wheat-midge 

 and the Hessian-fly, Diplosis tritici (Kirb.), arid 

 Cecidomyia destructor (Say.). The latter has 

 only lately been recognised as a British species. 

 It attacks the stems of the wheat, which crack and 

 bend over, and this appearance furnishes the easiest 

 and most reliable evidence of the presence of the 

 pest among the wheat. Its ravages are, however, much checked by the attacks 

 of various small parasites belonging to the Hymenopterous family Ghalcididce. 



Fig. 96. HESSIAN-FLY 



(Cecidomyia destructor). 



Magnified. 



