260 CLASS VIII. 
which are usually filled with air by external openings (st¢gmata). 
These canals have three coats: an external, loose, transparent 
membrane, in which fibres and scattered points (cell-nuclei) may 
be distinguished; a middle, composed of a flat, horny, sometimes 
brown or yellow elastic thread rolled spirally: and an inner coat 
which is composed of chitine, a continuation of the external skin, 
and is thrown off at every moult!. Through the elasticity of the 
spiral thread the air-canals are duly kept open: its turns lie close 
to each other, and so the appearance of rings is produced, as in the 
wind-pipe of mammals (this the representation of the trachex of 
Pediculus in SwAMMERDAM, Bibl. natur. Tab. I. fig. vil. resem- 
bles too closely); but the similarity is only in appearance; there 
are no absolute rings, but only the turns of a single uninterrupted 
thread. ach branch, arising from a stem, has a new thread, 
whether the branch proceeds laterally from the stem, or two branches 
arise at the end of the stem; this thread is finer than that of the 
stem, and in the terminal branches is only visible when very 
highly magnified. From being full of air, the canals, when 
Insects are dissected under water, have a silvery splendour, and 
present on account of the extreme fineness of their branching a 
very beautiful appearance to the observer?. Usually the air-canals 
divide, like arteries, into continually finer branches. In some 
Insects however there spring from a large stem on every side 
throughout a greater or less extent extremely fine and numerous 
branches (as ex. gr. according to Leon Durour, in Prionus, from 
the double stem which lies between the las€ stigma of the thorax, 
and the first of the abdomen). In Nepa and Ranatra saccules are 
seen in the cavity of the thorax, between which similar fine 
branches (retia mirabilia) of the air-canals le, and which are sur- 
rounded by a muscular coat®. Care must be taken to distinguish 
these saccules from the sacculated dilatations of the air-canals them- 
selves, which are met with in flying Insects in the last period of 
1 Tt has not been made out, as far as I know, whether the innermost membrane of 
the air-tubes is present in those insects also which have no stigmata, but gill-plates, as 
the larvee of Hphemera, for instance. 
2 M. Matpicur, who first made use of the names of trachee and stigmata, says, 
‘“‘ Tanta est fructificatio horum vasorum, tam mire implicationes ut nil pulchrius conspici 
possit.” De Bombyce, p. 12. Opera om. Tom. 11. Londini, 1687, fol. 
3 Léon Durour, Rech. sur les Hémipteres, p. 253, Pl. XVII. 
