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TEXT-BOOK OF ENTOMOL<><;Y 



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 

 seise, which form a lattice work or grate 



.eftThM 4 sdgma"ntChea e of !^ f to keep Ollt dust, dirt, fluids, etc. 



lunllni nilijti rix. showing the chamber 



or drum leading into the trachea Krancher 1 has described five leading types 



of stigmata, not, however, taking into account 

 those of the Synaptera. 



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 setse (Fig. 402). 



a, a, external frame or valve protect 

 ing the outer opening of the stigma 



b, c, c, inner frame closing the en 

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

 clusor muscle closing the inner orifice. 

 After Straus-Durckhcim. 



FIG. 402. First abdominal spiracle with a 

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

 t, trachea. 



FIG. 4o:>. Stisrma of NU-lolontha larva, seen 

 from without : b, bulla ; s, 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 larvae 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 

 changes, from Kolhc's work. 



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

 manently open, owinir to tin- absence of a valve, but communication with the trachea! 

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



