DIPT ERA. 255 



dibles (;;//'/) are modified into hooks for dragging the 

 insect along ; mx^ are the maxillae ; lb is probably the 

 labrum. PI. XII., Fig. 201, s\ is the prothoracic spir- 

 acle ; and PI. XII., Fig. 202, the spiracle enlarged. 

 The larval stage averages from five to seven days, 

 then the larval skin hardens and gradually separates 

 from the pupa within. This larval skin is known as 

 the puparium (PL XII., Fig. 204). Within this case 

 the coarctate pupa (PL XII., Fig. 205), as it is called, 

 transforms into a fly in five or seven days. The meta- 

 morphosis is completed, therefore, in the short period 

 of from ten to fourteen days. The mature insect 

 remains in a torpid state through the winter, and 

 appears the following May or June. It does not lay 

 its eggs till August, after which it dies. 



The Diptera include an immense number of fami- 

 lies, from which it is only possible to select a few of 

 the commonest and most instructive. The order has 

 been divided by Brauer into two large groups, — the 

 Orthorhapha and Cyclorhapha, — and this classifi- 

 cation is based upon larval and pupal characteris- 

 tics. In the Orthorhapha the larval skin opens at the 

 last moulting along the middle of the back, from the 

 second to the fourth segment, and to this longitudinal 

 fissure is joined, near the forward end, another short 

 cross-fissure, so that a T-shaped orifice is produced. 

 It also happens in some species that the pupa escapes 

 through a transverse rent between the seventh and 

 eighth abdominal rings. 



In the Cyclorhapha the larval skin hardens into a 

 puparium, which is seldom similar in form to the 

 larva, and the pupa escapes from this puparium 



