POSITION AND NUMBER OF SPIRACLES 439 



separate, and more or less branched. In caterpillars, the setae are so finely 

 branched as to form a loose felt, or sieve-like arrangement. 



e. The stigmata are round, with a very broad border and a concentric mid- 

 dle portion, the structure being complicated. The concentric middle portion is 

 pouch-like and bears the occlusor muscle. This form occurs in the larvae of 

 lamellicorn beetles, and can be seen with the naked eye, or with a lens, in 

 Oryctes, Cetonia, and Melolontha (Fig. 403). 



/. Over the outer opening of the spiracle is an incurved chitinous projection, 

 on one side of which the trachea takes its origin. It is thus in the Hymenoptera. 



Th# remarkable grate-like stigma of the lamellicorn larvae has the appearance 

 as if the outer closing plate or valve were impenetrable. The earlier observers 

 considered these stigmata to be open, but Meinert regards them as closed ; 

 Schiodte, however, has observed by pressing a preserved specimen of a Melolon- 

 tha larva the alcohol within passing out in drops, through the grate-like plate, 

 and hence he considers this a proof that the stigma is permeable (Kolbe). 



More recently (1893) Boas has examined the same structure in the same 

 species of larva as examined by Schiodte, and he finds it to be open only during 

 the process of moulting. He finds that on each side of the larva there are nine 

 short and wide stisunatic branches, each of which is shut off from the exterior by 

 a brown plate ; this consists of a reniform sieve-plate, and of a curved bulla 

 which fits into the cavity of the plate. The stigrnatic branch, however, is pro- 

 vided with a large external opening, which is homologous with the stigma, but 

 which is usually closed by the plate and bulla, and is only open during the moult- 

 ing ; at first it is circular, but later becomes a cleft. A transverse section shows 

 that the bulla is a simple tegumeutary fold, the outer chitinous layer of which 

 has become especially firm. The plate forms a horizontal half-roof, which 

 springs from one side of the tracheal orifice, and is supported by obliquely set 

 bases, which spring from the adjoining part of the inner side of the tracheae. 

 The plate and bars are purely cuticular structures. (Zool. Anz., 1893 ; also 

 Journ. Koy. Micr. Soc., p. 54.) 



The tracheal system of libellulid nymphs is not closed ; on the other hand, 

 in the fully-grown nymphs the anterior stigmata occurring on the dorsal side are 

 large, and the tracheae arising from them are thick. These stigmata are permea- 

 ble by the air. In half-grown and still younger stages of .ZEschna the two an- 

 terior thoracic stigmata are undeveloped. In order to breathe, the fully-grown 

 nymph either rises up on the upper side and elevates the end of the body to the 

 surface in order to take the air into the rectum, or it rests with the back of the 

 thorax at the surface in order to breathe through the large stigmata. The young 

 nymphs take in air only through the rectum. The young nymphs of Libellula 

 and its allies, on the other hand, possess large thoracic stigmata, but they prefer 

 to breathe through the rectum. The fully-grown nymphs of Agrion breathe 

 through the thoracic stigmata. (Dewitz, in Kolbe.) 



The position and number of pairs of stigmata. --The spiracles are 

 usually situated in the soft membrane between the tergites and 

 pleurites, but their exact position varies in different groups. In the 

 Coleoptera they occupy on the thorax a more ventral position, and 

 on the abdomen are placed near the edge of the dorsal side, under 

 the elytra. In the dragon-flies, the first pair is situated much more 

 dorsally than the second and third pairs ; the following seven pairs 

 are almost wholly ventral and lie concealed in the membranous fold 



