204 MAINE AGRICULTURAL EXPERIMENT STATION. 1916. 



considered that there are two head segments cephalad of the 

 anterior spiracles; and that these spiracles are borne on the 

 third or prothoracic segment. On the basis of the segmental 

 spines there are usually eight similar and sub-equal segments 

 caudad of the one bearing the anterior spiracles, and one ter- 

 minal segment which bears the posterior respiratory process, 

 making twelve in all. 



(3) The Pupa. When the larva is fully grown, commonly 

 about a half inch in length, it comes to rest in some suitable 

 place in or near the larval habitat and prepares for the trans- 

 formation to the winged fly. It fastens its caudal segments to 

 a leaf or twig or other support with glue from anal glands, or 

 buries in the ground. Its skin inflates dorsally and laterally, 

 nearly obscuring the wrinkles characteristic of the larva, 

 shortens on the ventral line and becomes much indurated by the 

 deposition of chitin, thus forming the pupa-case or puparium. 

 Within this case certain complicated changes take place during 

 which legs and wings and hair and eyes and mouth parts and 

 the other features of a winged fly, like the one that laid the 

 eggs, gradually make their appearance. 



Since the pupa is concealed in the puparium throughout this 

 stage, the characteristics readily observable are those of the 

 larval skin which are, except for shape, mostly the same as 

 during the larval period. The tracheae leading from both ante- 

 rior and posterior larval spiracles are atrophied during this 

 stage and respiration is provided for by a pair of stigmata which 

 pierce the puparium in the region of the fourth or fifth larval 

 segment. From each of these stigmata a large tracheal trunk 

 leads downward to the prothorax of the developing imago. In 

 some species the pupal respiratory stigmata are much elevated 

 on cylindrical cornua (Figs. $6-8 A; 3J-6A) ; in others they 

 are sessile, exceedingly small and often indistinguishable or 

 apparently wanting (Fig. 30-7^). The characteristics of the 

 larval skin, such as anterior and posterior larval spiracles, pro- 

 legs and vestiture, usually remain practically unchanged through- 

 out this period. 



The coarctate pupa (Fig. 32-5^, 54) is protected by the hard 

 puparium and, within this, a thin, delicate, transparent mem- 

 brane (Fig. 32-540) encases the developing wings, legs, and 

 other parts of the body. The segmentation and appendages of 



