(97) 



STETJCTUEE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 537 



first. The apparent single second segment is considered to 

 be of a double nature. 



8. The muscular system is described in detail. It consists 

 of : (1) A segmentally-arranged series of flattened cutaneous 

 muscles forming an almost perfect sheath below the hypo- 

 dermis ; (2) the muscles controlling the cephalo-pharyngeal 

 sclerites and pharynx; (3) the cardiac and visceral muscles. 

 The series of muscular actions which probably takes place 

 during locomotion is described. 



9. The central nervous system is concentrated to form a 

 single compound ganglion in which eleven pairs of compo- 

 nent ganglia can be recognised. On the dorsal side of the 

 anterior end of the ganglion two cerebral lobes united in the 

 median line above the cesophageal foramen are situated; 

 these contain the rudiments of the optic and supra-cesophageal 

 ganglionic structures of the fly. Eleven pairs of segmental 

 nerves arise from the ganglion, and in addition to these three 

 pairs of lateral nerves, and also a single pair and two median 

 unpaired dorsal accessory nerves arise. The component 

 ganglia are surrounded by a cortical layer containing large 

 ganglion cells ; the whole compound ganglion is enclosed in a 

 capsular sheath. 



The only sensory organs are two pairs of tubercles situated 

 on the dorsal sides of the oral lobes. By their structure they 

 indicate an optical function. 



10. The alimentary tract is very long in the larva, the 

 ventriculus being especially elongate. It consists of pharynx, 

 oesophagus, proventriculus, ventriculus, intestine and rectum. 

 In addition to a pair of salivary glands, whose ducts unite to 

 open by a single duct at the anterior end of the pharynx, and 

 a pair of bifurcating Malpighan tubes, the larva possesses four 

 caBca at the anterior end of the ventriculus. The ventri- 

 culus and intestine are very convoluted and are coiled up to 

 form a complicated visceral mass. 



11. The tracheal system of the adult larva consists of two 

 longitudinal lateral tracheal trunks united Jby anterior and 

 posterior commissures, and communicating with an exterior 



