of chitin, which are connected posteriorly with the occipital foramen. 

 The foramen itself is strengthened and defined above by a thick arch 

 of chitin, the extremities of which are turned outwards. The various 

 portions of the margins of the foramen have received distinctive names, 

 which are of little importance for the present purpose. The rigid 

 nature of the margins of the foramen should be remembered when 

 dissecting the part. 



The ptilinum is characteristic of the Cyclorrapha, and is related to 

 the method by which the imago emerges from the pupal case. The 



external opening, as already described, lies above and 



. f J The Ptilinum 



at the sides of the antennae, and is known as the 



ptilinal suture. From the margins of this orifice the wall of the head 

 cavity is invaginated in the form of a much convoluted membraneous 

 sac, the external walls of which are covered with scales of various shapes 

 (Plate XVI, fig. 2) ; the internal surface of the membrane has attached to it 

 a number of scattered muscle fibres. In sections the sac is found in the 

 upper part of the head and in front of the brain; its wall is always 

 thrown into very numerous folds, so that on a first examination one 

 would hardly recognize it as a sac. In newly-hatched flies it can be 

 easily evaginated as a rounded tumour by compressing the head with 

 a needle. 



The purpose of the ptilinal sac is to thrust off the end of the puparium 

 at the moment the fly is ready to emerge, and this is accomplished 

 by the distension of the sac with air, by respiratory movements of 

 the body. The method of emergence, and the consequent . presence 

 of the ptilinal suture in the imago, are two of the essential features 

 which separate the Cyclorrapha from the rest of the Diptera; the 

 Orthorrapha do not escape by detaching the end of the pupal case as 

 a circular cap, but through a T-shaped opening on the dorsal surface. 

 It is this feature which has given the flies the name Cyclorrapha. 



The neck is always well demarcated in the Diptera. In most flies it is 

 a simple membraneous tube connecting the head with the thorax, but in 

 some Orthorraphic flies the membrane may be strength- 

 ened by small lateral plates of chitin. These, how- 

 ever, are never sufficiently well developed either to impart any rigidity 

 to the neck, such as would enable the fly to use the muscular force of 

 the body to thrust its mouth parts into a resistant layer, or to interfere 

 with the free movements of the neck. The movements of the neck are 

 not extensive, and are limited to the adaptation of the proboscis to 

 the position requisite for feeding. They are brought about by certain 



