I40 INJURIOUS AND USEFUL INSECTS 



which are deep-seated, and were once supposed to be inde- 

 pendent of the larval tissues. They are now known to be 

 folds, either of the epidermis or of some other cellular layer. 

 The process is much more intelligible in Nemoceran larvae or 

 in Lepidoptera, than in the blow-fly, which is highly specialised 

 and peculiar, and forms indeed an extreme case of its kind. 

 During this complete regeneration of the body, the insect is 

 of necessity motionless. It is completely protected by the 

 puparium, or hardened larval skin. Respiration is carried on 

 through the posterior spiracles, which still remain in a 

 functional condition. 



If we remove the larval skin from a black pupa-case, we 

 shall find within it the true pupa, a whitish object, not unlike 

 a Lepidopterous chrysalis, with its legs and wings disposed 

 with perfect regularity on the sides of the body. All the 

 parts are covered by a transparent pupa-skin. 



The fly escapes by raising a kind of lid, composed of the 

 first four rings of the pupa-case. When fresh emerged, it is 

 soft and pale ; the wings are crumpled and useless for flight. 

 A large, bladder-like frontal sac projects from the head, and 

 it is probable that the expansion of this sac pushes off the lid 

 of the pupa-case. After a few hours the body of the fly 

 becomes distended with air, and darkens gradually to a blue- 

 black colour, the chitinous cuticle hardens, the wings expand, 

 and the insect becomes capable of flight. 



The Fly 



The head is largely occupied by the compound eyes 

 feelers or antennae, and proboscis. It is attached to the 

 thorax by a very slender neck. Examine the front of the 

 head with a lens, and observe the large compound eyes, 

 nearly meeting in the male, but more widely separated in the 

 female. Note that in male insects the eyes are often larger 

 than in the female. Why ? Observe also the three simple 

 eyes on the top of the head, partly hidden by hairs, and the 

 antennae, each with three enlarged basal joints and a 

 slender bristle, which bears a brush of fine hairs. The 

 antennae lie close together in a cavity which occupies the 

 middle part of the lower half of the face. The bristle projects 

 from the third joint near its base. A cavity will be found on 



