398 EXTERNAL ANATOMY OF INSECTS. 



minishing its powers of resistance in others, so that it 

 may yield somewhat to the action of the muscles ; in- 

 creasing or deducting from the weight of the body, so as 

 to produce a proper equipoise during its motions, whe- 

 ther on the earth, in the air, or in the water. The de- 

 pressions of the outer surface of the crust, in many in- 

 stances, produce an elevation of it in the interior, and 

 so afford a useful point of attachment to certain muscles. 

 This observation seems more especially applicable to 

 those excavations that are common to particular tribes 

 or genera : thus the dorsal longitudinal channel to be 

 met with on the prothorax of most of the Carabi of Linne 

 on the inside of the crust have a corresponding ridge. 

 In Locusta Dux, also, (a Brazil locust,) the same part 

 has four transverse channels, corresponding with which 

 on the inside are as many septa, or ridges, to which mus- 

 cles are attached; and those larger impressed puncta 

 denominated puncta ordinaria, which distinguish the 

 same part in Geotrupes and many of the Scarabceida, 

 within are elevated, so as to form a kind of ginglymous 

 articulation with the base of the anterior coxae. The 

 other impressed puncta so often to be seen on the diffe- 

 rent parts of various insects, which sometimes so intirely 

 cover the surface that scarcely any interval is discover- 

 able between them, though in many cases they appear to 

 be mere impressions that attenuate but do not perforate 

 the crust — yet in others, perhaps equally or more nu- 

 merous, they are real pores, which pass through the in- 

 tegument. If, for instance, you take the thoracic shield 

 of the cockchafer (Melolontha vulgaris), and after re- 

 moving the muscle &c, hold it against the light, with 

 the inner side towards the eye, you will see the light 



