160 AXATOMY. 



relaxes, it re-opens. We must observe, at the same time, that a bag- 

 shaped expansion of the tracheae originates from the circumference of the 

 spiracle, and narrows towards the latter, in a funnel shape. By means 

 of the tracheae arising from the point of the funnel, the whole expansion 

 is drawn backwards, so that the axis of the funnel stands obliquely to 

 the axis of the tracheae ; upon the inner side of this funnel, or that 

 part next to the ventral cavity, the just described apparatus for the 

 closing of the spiracle lies (see PI. XXIII, f. 1 3). Such spiracles are 

 found only upon free or slightly covered parts of the body, for example, 

 under the elytra of many beetles. 



A third form of the spiracles is distinguished from the preceding by 

 the want of lips. In very small and round spiracles, the opening is free 

 (for example, in the Lamellicornia), or at most covered with short hair 

 upon their inner margin, and the entrance into the tracheae is only 

 rendered difficult by the obliquity of its axis to that of the spiracle. 

 In larger oval spiracles, the margins are occupied with stronger plumose 

 spines, or hairy tufts (PI. XXII. f. 10), and these resist extraneous 

 substances still more forcibly. The air is purified through these as 

 through a sieve, and all prejudicial substances are caught there. This 

 structure is very distinct in the large spiracle of the first abdominal 

 segment of the male Cicada, as well as in the dorsal spiracles of the 

 water beetles *. 



The fourth and last form of the spiracles is that observed in the 

 larvae of the Lamellicornia. In these the very minute spiracle appears 

 at first view to take a circular shape, and upon closer inspection it is 

 found to consist of a broad margin and a concentric middle space, 

 which beneath breaks through the margin and connects itself with the 

 surrounding integument. This margin, which is often ornamented 

 with distinct sculpture (PL XXIII. f. 4. a, a,) Sprengel considered as 

 a half moon-shaped opening, occasionally closed by a sieve, when the 

 sculpture of the margin was cribriform, or by toothed processes, when 

 the sculpture took that figure, opposite which the inner round plate lay 

 and assisted to close it. Treviranus t opposes this view of it, and asserts 

 that the spiracle is entirely closed, but that minute ramifications of 

 tracheae are spread upon its internal superficies, and imbibe the air, 



* See Cams, Analekten zur Natunvisscnscli. Dresden, 1829. 8vo. P. 187. PI. I, 

 f. 13. And Sprengel, Commentar. &c. Plate II, fig. 23; and Plate III, fig. 29. 

 ) Das Organische Leben, nen dargestellt. Bremen, 1831. 8vo. Vol. I. p. 2.58. 



