282 



ORTHOPTEPA 



CHAP. 



Fig. 168. Hind leg of Porthetis sp. 

 Transvaal. 



(Fig. 185, C). The hind legs are occasionally very long; their 

 femora, thicker towards the base, are generally peculiarly sculp- 

 tured, bearing longitudinal ridges or grooves, which are more 



or less spinose, and are also 

 very frequently marked with 

 short parallel lines meeting a 

 central longitudinal line at 

 similar angles, so as to give 

 rise to a well-marked pattern ; 

 where the legs are broader 

 the pattern is more complex 

 (Fig. 168). The long tibiae 

 bear two rows of spines on 

 their upper or posterior edge ; 

 this part of the hind leg can 

 be completely bent in under 

 the femur. The stigmata consist of one pro thoracic, one meta- 

 thoracic, and eight abdominal pairs. 



In reference to the ocelli, which are shown in Fig. 166, we 

 may remark that the Acridiidae is one of the large groups of 

 Insects in which the coexistence of compound and single eyes is 

 most constant, though in some of the wingless forms the ocelli 

 are very imperfect. We know at present of nothing in the 

 habits of Acridiidae to render two kinds of eyes specially neces- 

 sary. We shall subsequently see that a similar condition in 

 regard to the function of hearing is believed to exist in this 

 family. . 



Acridiidae are remarkable amongst the Orthoptera for the 

 possession of air sacs or vesicular dilatations in the interior of the 

 Insect in connexion with the tracheae (Fig. 176). Such vesicles 

 are found in many of the higher winged Insects, but not in larval 

 forms, or in those that are destitute of powers of llight.^ They, 

 no doubt, assist the Insect in its movements in the air. The body 

 of a large grasshopper or locust is naturally of considerable 

 weight, and it is more than probable that true flight can only be 

 accomplished when these vesicles are dilated and filled with air. 

 The exact mode in which the sacs are dilated is not known ; 

 possibly it may be accomplished by the elasticity of the structure 

 of the vesicles coming into action when the other contents of the 

 1 Newport, Tr. Linn. Soc. xx. 1851, p. 419. 



