STRUCTURE,|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 seementally-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. Hleven 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 
oanglia 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 alhmentary tract is very long in the larva, the 
ventriculus being especially elongate. It consists of pharynx, 
cesophagus, 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 
ceca 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 by anterior and 
posterior commissures, and communicating with an exterior 
