MEMBRACIDAE. 
9 
tubes rising directly from the surfaces, and occur of all shapes and sizes. If broken 
off, they leave clear apertures in the integument. Being filled with air, they do not 
impede the movements of the insect, as they would do if solid. 
The chitinous parts resist the action of caustic potash solutions, and thus they 
behave quite differently from the substance of skin, feather and horn.* 
Ingenious speculations have arisen as to the elimination of carbon by insects through 
their respiratory systems. Some have thought that the curious chitinous excrescences 
of beetles and other insects partake of the nature of excrementitious matter thrown 
off by the tracheae. As this matter is assumed commonly after the time of pupation 
it might be thought that the waste matter accumulates, from the inability of the 
young imago to throw it oft’, and that the growths take place from the centres of least 
resistance, branching into forms like those of Sphongophorus. 
Tracheal sacs however are common in many larvae, where no free carbon is 
eliminated, and it is difficult to regard the grand framework of the Coleoptera and the 
Membracidae as derived from mere waste product. 
The exact composition of chitin is yet obscure. After the soft parts of insects 
have been removed by solution in dilute caustic potash, the remaining part may be 
readily bleached by a few drops of hydrochloric acid into which some small fragments 
of potassium chlorate have been thrown. Mounting in Canada balsam then will dis¬ 
close many details not otherwise visible. 
The large pronotal processes of Membracidae must seriously affect the stability of 
the insects duing flight, but as all the specimens I have examined show these processes 
as hollow chambers, the wings have less work to do to keep the insects poised in 
horizontal positions. 
The formation of pattern by stain in the substance of cbitin is of great interest, but 
the question involves too many questions for consideration in this place. Why pattern 
should be so constant in some genera, and so variable in others, is as yet very obscure. 
Patches of white or of colour appear in some Membracidae to be due to a peculiar 
felt-like surface resembling fine flannel. This is a growth on the chitinous coat. It 
appears on the wings of many Cicadidae and Fulgoridae, and is quite distinct from the 
scales on the wings of the Lepidoptera. In the first mentioned families many genera 
might easily be mistaken for moths or sphinges. 
In Centrotus and Pterygia, &c., amongst the Membracidae, this adherent pile is 
often scattered in isolated patches with pronounced pore marks, like black spots in 
white ermine. 
The Head .—As a rule the head is not large. It is mostly prone, that is, it is so 
sloped, that the insect looks downwards on the twigs which support it. 
* See Bataillon on Chitin , Bull. Sci. France Belgique, xxv. 1893, p. 18. 
B 
