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TEXT-BOOK OF ENTOMOLOGY 



FIG. 415. Muscles of right half of the abdomen of For- 

 (luricularia : A, a, longitudinal tergal and sternal mus- 

 cles ; D, , oblique muscles ; a (.in upper figure) vertical expirator 

 muscles. 



the respiratory organs. The result at which he has arrived may be summarized 

 as follows : 



" 1. There is no close relation between the character of the respiratory move- 

 ments of an insect and its systematic position. Respiratory movements are 

 similar only when the arrangement of the abdominal segments, and especially 

 when the disposition of the attached muscles, are almost identical. Thus, for 



example, the respiratory 

 movements of the cock- 

 roach are different from 

 those of other Orthop- 

 tera, resembling those of 

 the heteropterous Hemip- 

 tera. Those of the Tri- 

 choptera are like those 

 of the aculeate Hymen- 

 optera, while the Locus- 

 tidse ally themselves in 

 respect to these move- 

 ments with the Neurop- 

 tera and Lepidoptera. 



"2. The respiratory movements of insects, when at rest, are localized in the 

 abdomen. As graphically stated by Graber, in insects the chest is placed at 

 the hinder end of the body. If thoracic respiratory movements exist, they do 

 not depend on the action of special muscles. 



u 3. In most cases the thoracic segments do not share in the respiratory move- 

 ments of an insect at rest. The respiratory displacements of the posterior seg- 

 ments of the thorax are, however, less rare than Rathke believed. Plateau has 

 observed them in certain Coleoptera (Staphylinus, Chlorophanus, Corymbites). 

 and they are more feebly manifested in Hydrophilus, Carabus, and Tenebrio. 

 Among the singular exceptions to this rule is the cockroach (Periplaneta 

 orientalis}, in which the terga of the meso- and metathoracic segments perform 

 movements exactly opposite in direction to those of the abdomen (Fig. 419). 



"4. Leaving out of account all details and all exceptions, the respiratory 

 movements of insects may be said to consist of the alternate contraction 

 and recovery of the figure of the ab- 

 domen in two dimensions, viz. vertical 

 and transverse. During expiration both 

 diameters are reduced, while during 

 inspiration they revert to their previous 

 amounts. The transverse expiratory 

 contraction is often slight, and may be 

 imperceptible. On the other hand, the 

 vertical expiratory contraction is never 

 absent, and usually marked. In the 

 cockroach (P. orientalis) it amounts to 

 one-eighth of the depth of the abdomen (between segments 2 and 3) ; in 

 Eristalis tenax to one-ninth (at the 2d segment). 



"5. Three principal types of respiratory mechanism occur in insects, and 

 these admit of further subdivision : 



" a. Sterna usually short and very convex, yielding but little. Terga mobile, 

 rising and sinking appreciably. To this class belong all Coleoptera, heteropte- 

 rous Hemiptera, and Blattina (Fig. 420). 



" In the cockroach (Periplaneta), the sterna are slightly raised during expira- 

 tion (Fig. 421). 



FIG. 41fi. Muscles of the left half of ab- 

 domen of Staphylinus nfeiift: A, , longitu- 

 dinal dorsal muscles; D, E, oblique fascia; 

 a, longitudinal sternal muscles ; d, respiratory 

 muscles (vertical expirators). 



