MOTIONS OF INSECTS. 357 



not visited by some of its numerous tribes. The four 

 wings of these insects, the upper pair of which are 

 larger than the under, vary much in their nervures. 

 From the saw-flies (Tenthredinidce), whose wings are 

 nearly as much reticulated as those of some Neuro- 

 ptera^ to the minute Chalcis and Psilus, in which these 

 organs are without nervures, there is every interme- 

 diate variety of reticulation that can be imagined'*. It 

 has been observed, that the nervures of the wings are 

 usually proportioned to the weight of the insect. Thus 

 the saw-flies have generally bodies thicker than those 

 of most other Hi/menoptera^ while those that have 

 fewer nervures are more slender. This, however, does 

 not hold goi'd in all cases — so that the dimensions and 

 cut of the wings, the strength of their nervures, and 

 the force of their muscles, must also be taken into con- 

 sideration. The wings of many of these insects when 

 expanded, are kept in the same plane by means of 

 small hooks {Hamuli) in the anterior margin of the 

 under wing, which lay hold of the posterior margin of 

 the upper ''. Another peculiarity also distinguishes 

 them. Base-covers ( Tegulce), or small concavo-convex 

 shields, protect the base of the wings from injury *=, or 

 displacement. 



The most powerful fliers in this order are the humble- 

 bees, which, like the dung-chafers (Scarabceus), traverse 

 the air in segments of a circle, the arc of which is alter- 

 nately to right and left. The rapidity of their flight is 

 so great, that could it be calculated, it would be found, 

 the size of the creature considered, far to exceed that 



* Jurine Hymenopt. t. 2-5. " Kirby Mon. Ap. Angl. i. 96, 108. 



«. xiii./. 19. ■= Ibid. 96. 107. <. v./. 8. rfd. 



