INTRODUCTION. 239 



and lower surface, there is also remarkable uniformity. They 

 are generally elongate insects, sliort-legged and parallel-sided. 

 A few, however, have the hind body shorter and less jiarallel, 

 and this is an indication of atrophied wings and loss of the 

 power of flight. Tiiis is the case with one Indian sp., Macro- 

 linus obesus. Plenrarius brachyphyUn-'^- is said by Gravely to be 

 incapable of flight also, although the wings are j)erfectly 

 developed and the shape of the hind-body has not undergone 

 alteration. The loss of the faculty is no doubt much more 

 recent in this case. In tropical America a considerable 

 number of different species are found of which the wings are 

 reduced to narrow, but rigid strips of membrane, and the wliole 

 shape of the insects has become altered in correspondence. 

 The explanation appears to be that in all the Pas.salidje the 

 wings have accjuired a secondary function, as jiart of the 

 apparatus for voice production, which is of greater im])ortance 

 than the primary fuiiction of flight. There is a consequent 

 tendency for the more important function to be improved by 

 the sacrifice of the less important. No other family of beetles 

 is known in which sound-producing organs are found in botli 

 adults and larva? of every species. 



The beetles produce a scjueaking noise by rubbing the terminal 

 part of the abdomen against the wings when these are lying 

 folded beneath the elytra and presse<l against their inner 

 surface. The third dorsal segment of the aljdomen from the 

 end bears upon each side a rounded eminence with a pecuharly 

 roughened and exceedmgly hard surface. Each of these 

 bosses coincides with the homy patch which occurs at the 

 part of the front margin of the wing where it folds back when 

 lying at rest, and these horny patches have also a peculiarly 

 roughened surface. The rubbing together of these two hard 

 rough surfaces produces the " voice " of the insect. In those 

 forms in which adaptation to this purjiose has gone farthest the 

 wing has become reduced to a narrow strip, reaching only as 

 far as the corresponding boss upon the back of the abtlomen, 

 and is hard and rigid. It lies in a depression of the wing-cover, 

 with a slight cavity behind it, Avhich perhaps increases the 

 volume of sound produced, like the space behind the stretched 

 parchment of a drum. 



One of the most distinctive features of the group is found in 

 the mode of articulation of the fore- and hind-l)ody. The 

 mesothorax is produced into a tube upon wliich the pio- 

 thorax moves freely in any direction, the base of the pionotum 

 not fitting closely to the bases of the elytra. The scutellum 

 does not, as usual, project between the two eh-tra and is 

 capable of being completely covered by the pronotum. The 

 legs are adapted for digging and furnished with sharp teeth and 

 spines. The coxae are deeply imbedded, the front tibia armed 



