STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 513 
are two prominent anal lobes (fig. 5, av. 1.), which are impor- 
tant agents in locomotion. 
The cuticular integument is thin and rather transparent, so 
that in the younger larve many of the internal organs can be 
seen through it. In older larve the fat-body assumes large 
proportions and gives the larva a creamy appearance, obscur- 
ing the internal organs. The cuticle (fig. 14) is composed of 
an outer rather thin layer of chitin (ct.), which is continuous 
with the chitinous intima of the trachez, and also with the 
chitinous lining of the stomodieal and proctodeal regions of 
the alimentary tract. Below this layer there is a thicker 
layer of chitin (ct.’), which does not stain so deeply. In 
places this lower layer is penetrated by the insertions of the 
muscles. The cuticle lies on a layer of stellate hypodermal 
cells (hy.), which are well innervated, and attain a large size 
in the posterior segments of the body. 
2. MuscunarR System. 
The muscular system of the larva (PI. 31, fig. 16) consists of 
a segmental series of regularly repeated cutaneous muscles, 
forming an almost continuous sheath beneath the skin, 
together with a set of muscles in the anterior segments of 
the body which control the cephalo-pharyngeal sclerites and 
pharynx. In addition to this there are a set of cardiac 
muscles and the muscles of the alimentary tract. 
I have been unable to find a detailed description of the 
muscular system of the muscid larva, and I do not think that 
Lowne’s excuse for dismissing the cutaneous muscles of the 
blow-fly larva with a very brief statement, because “the 
details possess little or no interest,” was justified, considering 
how little is known about the muscular systems of insect 
larvee, and constant reference to the classic work of Lyonet 
(1762) on the caterpillar is not sufficient to satisfy the 
inquiring student of to-day. The muscular system of the 
larva, therefore, will be described in some detail. 
Muscles of the body-wall.—The cutaneous muscles 
