508 THE INVERTEBRATA 



chitinous teeth which make the wound. Thus a second kind of blood 

 sucking mechanism has been evolved from a form like Musca, which 

 only possessed the faculty of sucking fluid from surfaces. 



The larvae of Diptera are among the most specialized in the Insect 

 Kingdom. Legs have been entirely lost, and the head and spiracular 

 system have undergone varying degrees of reduction. Thus the most 

 generalized larvae are at the same time eucephalous, i.e. with complete 

 head capsule, and peripneustic ^ i.e. with lateral spiracles on the ab- 



B 



Pig. 2^f). Early stages of the Diptera. A, Larva of Musca domestica. Ace- 

 phalous amphipneustic type. B, Empty puparium of Musca domestica. 

 C, Pupa of Musca domestica removed from puparium. D, Larva of Bibio sp. 

 Eucephalous peripneustic type. A, B, and C after Hewitt; D, original. 

 sp. spiracle. 



domen, e.g. Bihio (Fig. 349 D). In the most specialized forms, on 

 the other hand, we find the acephalous larva whose head capsule is 

 entirely wanting. Such acephalous larvae may be either amphi- 

 pneustic^ with only prothoracic and posterior abdominal spiracles, or 

 metapneustic, where only two spiracles are retained at the posterior 

 end of the body. The first instar larva of Musca is metapneustic, 

 subsequent instars being amphipneustic (Fig. 349 A). 



The eucephalous larva develops into an exarate pupa from which 

 the adult emerges by a longitudinal slit on the thorax. The pupa 



