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TEXT-BOOK OF ENTOMOLOGY 



tubes. In its essential structure a spiracle, or stigma, is a slit-like 

 opening surrounded by a chitinous ring, the lips or edges of the 



opening being membranous and closed by 

 a movable valve of the spiracle attached 

 by its lower edge, which is closed by an 

 occlusor muscle (Fig. 401). The aperture 

 when open forms a narrow oval slit ; 

 and in most insects the slit is within 

 guarded by a row of projecting spines or 

 setae, which form a lattice work or grate 



FIG. 401. Horizontal section of j T,-,-, vnf rliicf rlirf flniilc pff 



left third stigma and trachea of Melo- tO Kee P OUt aust > ant > HUKIS, CIC. 

 lontha vulaaris. showing- the chamber 



or drum leading into the trachea: Krancher * has described five leading types 



</, , external frame or valve protect- of s tigmata, not, however, taking into account 

 ing the outer opening of the stigma ; 



li, c, o, inner frame closing the en- those of the Synaptera. 



trance into the trachea (/, k) ; m. oc- 

 clusor musc.le closing the inner orifice. 

 After Straus-Durckheim. 



I. Stigmata without lips (Primitive or gener- 

 alized stigmata). 



a. The simplest stigma is an aperture which 



is kept open by a chitinous ring (Acanthia). The opening may be round or 

 elliptical. There are no lips nor any movement of the edges to be observed. 

 Such air-holes occur in the abdomen of bugs (Hemiptera) and beetles (Cole- 

 optera) ; within the opening of the stigmata in the same insects is a funnel-like 

 contraction. Also in the Diptera the abdominal stigmata are of the same type. 2 

 The stigmata of the Pulicidae (Siphonaptera) are more complicated, as the 

 edges of the openings are provided 

 with setae (Fig. 402). 



FIG. 402. First abdominal spiracle with a 

 part of the trachea of the cat-flea : sp, spiracle ; 

 t, trachea. 



FIG. 4U8. Stigma of Melolontha larva, seen 

 from without: b, bulla ; *, sieve-like plate; o, 

 curved slit-like opening. After Boas. 



b. The stigma consists of a series of minute single stigmata, which are usually 

 surmounted by a common chitinous ring, and whose tubular continuations unite 

 within in a common trachea, so that the single tubes pass off from the stigma 

 like the fingers on the hand. This form is found in the larv;e and puparia of 

 Diptera. 



II. Stigmata with lips (Secondary more specialized stigmata). 



c. The lips are represented by a single chitinous ring, with sparse spines. 

 One side of the stigma is a little higher, and partly overlaps the other posteriorly ; 

 this form is peculiar to the Orthoptera and Libellulidae. 



d. The lips are roof-like, bent inwards and densely hairy, forming a peculiar 

 kind of felting. The setae of the lips are in most beetles and many Lepidoptera 



1 The following summary compiled from Krancher, is translated, with some minor 

 (lump's, from Kollxi's work. 



2 Miall and Denny state that in the cockroach the abdominal spiracles are per- 

 manent ly open, owing to the absence of a valve, but communication with the trachea! 

 trunk may be cut off at pleasure by an internal occluding apparatus. 



