292 
POPULAR SCIENCE REVIEW. 
pondages ; the oesophagus extends from the mouth to about the 
third ring of the thorax, it is without folds, and surrounded at 
the upper part by the oesophageal ganglionic ring, gradually 
leading to the crop, from which it is separated by a small, thick, 
circular valve. The crop, or ventricle, extends from the third 
segment of the thorax to between the fifth and sixth of the 
abdomen, is very extensible, and is frequently found greatly 
distended with dark food ; it has numerous transversal, nearly 
equidistant, folds, discernible by the naked eye, and several 
longitudinal muscular fibres ; at the bottom of the crop are 
inserted the hepatic vessels. The intestines are short, occupying 
the seventh, eighth, and ninth abdominal segments ; they con- 
sist of two parts, the upper portion corresponds to the small 
intestines in other animals, and is very short ; it is separated, by 
a little valve, from the large intestines, and is without folds ; 
the large intestines are three times the diameter of the small 
ones, and occupy the eighth and ninth segments. The five 
biliary appendages are very fine and of considerable length; 
they enter the intestinal canal at the bottom of the crop, and 
are generally whitish or rose-coloured, forming a labyrinth of 
entangled threads. 
The respiratory apparatus of the larva of a Fkvyganea con- 
sists of two large lateral tracheal trunks, which give forth 
numerous branches to the different parts of the body, supplying 
air to the muscles and the viscera ; if a portion of the muscular 
flesh be cut away, or if it be pressed between two bits of glass, 
we may see a tracheal filament or tv/o entering each external 
branchial appendage. The circulatory system probably differs 
in no essential respect from that of many other larvae, but it is 
extremely difficult to make out with satisfaction. On opening 
a larva we notice two long bright tortuous threads, of consider- 
able diameter, situated below the intestinal tract, and terminating 
in a coecum ; these threads double on themselves three or four 
times, and extend to about the fifth segment of the abdomen ; 
towards the anterior portion of the body they become of small 
diameter, and passing along the sides of the oesophagus, open 
out into the mouth. These are the vessels that secrete the 
silky substance by means of which the larvse cement together 
the materials which form their abodes. When the larva has 
assumed its pupa condition these secreting organs disappear. 
The perfect insects make their appearance in the spring, 
summer, and autumn, at which latter season great numbers 
may be seen near every stream and pond. Most of the species 
are small, or of moderate size, seldom more than two inches in 
the expanse of wings. Their mode of flight, as a rule, is after 
a zig-zag fashion, and some fly with considerable activity ; some 
species may be seen rising above the surface of the water in 
