338 INSECTS. 



or contracts into a smooth, egg-like case or cocoon. The 

 pupa itself, whether naked or enclosed in the larva skin, 

 resembles that of the Beetles in having the limbs sepa- 

 rately cased. 



The manner in which Flies are produced varies. Most 

 are, like other insects, produced in the egg state ; others, 

 among the Carrion Flies, are born, not in the egg, but 

 already grown to larvae ; while, in the case of certain 

 Parasitic Flies, they even attain the pupa stage before 

 exclusion. 



As in all the four-winged orders of insects, some 

 species or sexes are found wanting one or both pairs of 

 wings,* so in the order Diptera, characterized by the 

 invariable absence of hind-wings, the fore as well as the 

 hind pair are sometimes wanting, and also the halteres 

 or representatives of the hind-wings. 



If it be asked how, when, as in Diptera, or excep- 

 tionally in any of the orders above named, there is but 

 one pair of wings, they can be pronounced to be hind- 

 wings or fore-wings ? the answer is, much in the same 

 way as if a monstrous horse were born with only two 

 legs, it would be decided whether these were the hind or 

 the fore legs ; their relation to other parts would settle 

 the question. 



The order Diptera is divided by marked characters 

 into two sections, PROBOSCIDEA and EPROBOSCIDEA, but 

 of these the first contains the bulk of the order, while 

 the second contains only a few known species, which 

 are all parasitic, living in the perfect state on the 

 surface of the bodies of quadrupeds and birds. 



The characters of PROBOSCIDEA are proboscis fleshy 



* When only one is absent, it is nearly always the hind pair. 



