406 NATURAL HISTORY OF ARTHROPODS. 



The abdomen varies much in shape, being short, broad, slender, elongate, or 

 even wasp-like. At the tip the male organs (hypopygium) are often small, at other 

 times large, conspicuous, and complicated. In the female the last two or three 

 segments form a simple ovipositor, in most small and retractile, but not infrequently 

 elongate, and it may be as long as the entire body. Usually the eggs are deposited on 

 the surface of such substances as will serve for the future food, or are easily penetrable 

 by the larvae, and hence it is rare that any thickening of the ovipositor .or terminal 

 segments is necessary. 



On one side of the oesophagus there is a sucking stomach, as in the lepidoptera, and 

 there are four, or rarely live, Malj)higian glands corresponding to the kidneys, which 

 discharge the secretion into the intestine through one or two common outlets. The 

 tracheae, by which they breathe, consists of two bladder-like air sacs, situated one on 

 each side of the base of the abdomen. The nervous system is composed of a chain of 

 ganglia, in some species numbering nine or ten, while in others, the more specialized 

 and highly developed, the whole nervous system may be confined to a single ganglion 

 in the thorax, from which nerves proceed to the abdomen. 



The larvae are usually wholly footless maggots, moving by contraction and exten- 

 sion of the segments, or by leaping, as in the cheese-fly. They never have distinctly 

 differentiated thoracic legs, although often with a pair of protuberances on the first 

 segment, or, as in the Cecidomyidie, there may be a single unpaired hardened chitin- 

 ous spot on the third segment, that serves as means of locomotion. There are often 

 false abdominal legs that may be either distinct or merely swellings covered with 

 bristly booklets. They are either wholly headless, with an oral opening alone, or the 

 head may be partly or completely differentiated, with the mouth-parts rudimentary or 

 complete. The eyes, which when present are always simple and inconstant in their 

 position, are often wholly wanting. 



The metamorphosis is complete, and takes place chiefly in two different ways, 

 which have so far furnished the best general division for the order, and yet one that is 

 not wholly free from objections. In the first division, the Orthorhapha, which in- 

 cludes all the nematocerous flies, and some other families, the pupae may be either 

 free, as most usually is the case, or included in the larval skin, — the so-called puparium, 

 or the larva pupigera, — but, except in the pupigerous Cecidomyidae, the larval skin 

 when it bursts does so in a longitudinal rent on the back of the front end, and with 

 another, a transverse one, forming a T-shaped opening. In the other group, which 

 are always pupigerous, the perfect insect escapes from the larval skin through a more 

 or less circular opening at the anterior end, composed of the first two or three segments, 

 and forming a sort of a lid or cover. This sub-order is thus called Cyclorhapha. The 

 flies force the opening of this larval envelope by means of a large bladder-like inflation 

 or swelling on the front, and such flies have a curved space, the frontal lunule, imme- 

 diately above the antennse, that is wanting in the other group. This division includes 

 the larger part of the brachycerous flies. 



The larvae, as in the adult stages, breathe by means of stigmata, small openings fre- 

 quently placed along the sides, two on each segment ; but in a larger number, includ- 

 ing all the legless maggots, they are situated near the end of the body. In some, as in 

 also some of the larvae with heads, the stigma terminates in an elongated tube at the 

 hind end ; such are aquatic in habits. 



In the pupa stage the legs are not movable, or, if they are, the pupae are not free 

 but are included in the larval skin, which, by contraction, forms a free, loose envelope, 



