THE CRANE FLY. Ce | 
posed of a substance called chitin, resembling horn in its physical 
properties and forming a resisting outer case or exoskeleton, but 
hard internal parts are absent. 
The internal organs of Tipula comprise a heart in the shape of 
a long contractile tube on the back or dorsal side of the body, an 
alimentary canal with accessory salivary and hepatic glands, and a 
double chain of nervous ganglia placed. on the lower or ventral 
side of the body, with one large ganglion or assemblage of ganglia 
in the head above the cesophagus, and answering in function to the 
brain of higher animals. Respiration is carried out by branched 
tubes (trachez) which carryair all over the bodyand limbs, and whose 
main branches communicate with a pair of openings (spiracles) in 
each segment of the body, except those of the head whose tracheze 
spring from spiracles at the sides of the neck. The collapse of the 
tracheze by accidental pressure is prevented by a spiral thickening 
of their chitinous lining. 
The head of Tipula is elongated in a vertical direction, as shown 
in the plate, which represents a front presentation as it is seen in 
the accompanying preparation, but it must be pointed out that the 
antennz and maxillary palpi are necessarily constrained by the 
cover glass to assume unnatural positions. In life the antennze 
would be extended in the line of sight towards the observer—that 
is at right angles to the plane of the paper, and the maxillary palpi 
would be carried backwards under the head. ‘The first objects to 
claim attention are the eyes. Examined under a 1-inch objective | 
in a strong reflected light their surfaces will present a hexagonal 
(sometimes square) areolation, like those facetted lenses or spy 
glasses which in our childhood days amused and puzzled us with their 
multifold images. The further study of the insect eye must be 
made by means of vertical sections. It will then be seen that each 
hexagonal area or corneule, is a doubly convex transparent body, 
which must therefore act as a lens, and there is reason to believe 
that, in some insects, at all events, each corneule is composed of 
two halves of different density, whereby the production of false 
colour may be avoided. Behind the corneule, and separated from 
it by a ring of pigment, which answers the purpose of an iris, in 
limiting the path of the rays to the central portion, is a transparent 
cone with convex ends, placed with its base outwards or next the 
iris and its apex in connection with a single fibre of the optic 
nerve. All these structures go to make up a single eyelet or 
ocellite, and each is separated from its neighbour by a layer of 
dark pigment. The whole assemblage of a thousand or so 
ocellites, arranged side by side, with their apices converging to one 
point and their bases forming the external corneal surface, go to 
make up the single compound eye. The hexagonal form is the 
result of mutual pressure. Now, though it has been proved that 
