DIPT ERA 



287 



and are footless; some possess a distinct head, others do not; and there are 

 remarkable variations in the form of the respiratory organs, especially as 

 to the number and position of the spiracles. The pupa? are usually 

 either naked or enclosed in the last larval skin. A few are enclosed in 

 cocoons. When the pupa state is passed within the last larval skin the 

 body of the pupa separates from the larval skin more or less completely ; 

 but the larval skin is not broken till the adult fly is ready to emerge. In 

 this case the larval skin, which serves as a cocoon, is termed a puparium. 

 In some families the puparium retains the form of the larva; in others 

 the body of the larva shortens, assuming a more or less barrel-shaped 

 form (Fig. 553), before the change to a pupa takes place. 



This is a large order, both in number of species and individuals. The 

 species differ much in habits. Some are very annoying to man. Famil- 

 iar examples are the mosquito, which attacks his person; the flesh-flies 

 which infest his food ; the bot-flies and gad-flies that torment his cattle ; 

 and the gall-gnats that destroy his crops. Other species are very benefi- 

 cial. Many of the Syrphidas, and Tachinidse destroy certain noxious 

 insects; and other species, while in the larval state, feed upon decaying 

 animal and vegetable matter, thus acting as scavengers. 



Although the habits of some of these creatures, which revel in all 

 kinds of filth, are very disgusting, we cannot help admiring that arrange- 

 ment by which a mass of filth, instead of being left to poison the atmos- 

 phere, is transformed into myriads of living beings, whose swift flight and 

 delicate forms lend life and beauty to the landscape. 



CLASSIFICATION OF THE DIPTERA 



In the following table for determining the families of the Diptera, use 

 is made chiefly of characters based on the form of the head, antennae, 

 and wings. 



The more important of the charac- 

 ters presented by the head are the pres- 

 ence or absence of the frontal lunule, and 

 the presence or absence of. the frontal 

 suture when the lunule is present. 



The frontal lunule is a small crescent- 

 shaped piece immediately above the an- 

 tennae, which is characteristic of the 

 second suborder, the Cyclorrhapha. In 

 most of the members of this suborder 

 there is a suture separating the lunule 

 from that part of the head above it, the 

 frontal suture; and frequently this suture 

 extends down on each side to near the 

 mouth (Fig. 489). But as the suture is 

 wanting in several families of the Cyclor- 

 rhapha, it is often difficult to determine 

 whether the lunule is present or not. 



In those families that possess the 

 frontal suture there exists a large blad- 

 der-like organ, the ptilinuni which is 

 pushed out through this suture when the adult is about to emerge from 



Fig. 489. — Head of a fly: A, antennae; ar, 

 arista; E, eye; /. /., frontal lunule; f.s., frontal 

 suture. 



