THE ORASSIIOFPEB 



21 



places, such as Ijeneath stones or in holes which they make in 

 the ground. Their eggs are laid loosel.y in their retreats. 

 They make a familiar chirping noise, the blended sounds 

 of which, as heard on a summer evening, rise and fall in a dis- 

 tinct rhythm. The rate of chirp seems to be entirely deter- 

 mined by temperature, so that in the case of the " tree-cricket " 

 one may compute the temperature by means of the formula 



T = 50 -I- 



A^-40 



, in which T stands for temperature in degrees 



Fahrenheit, and A^ the numl^er of chirps per minut(\ The 



mechanism l^y which the chirp is produced is as follows : Near 



the midtUe of each of the ujjper 



wings of the male cricket is a 



vein of such roughness as to 



form a sort of file, and near the 



edge of the wing is a thickened 



scraper. When the upper 



wings are brought in contact 



above the body, and the scraper 



of one is rubbed across the file 



of the other, the wings are set 



in vibration, producing the call. 



A peculiar form of Gryllidis 

 is the mole-cricket (Fig. IG), 

 whose fore feet have become 

 much modified for burrowing. 



The Orthoptera constitute 

 one of the lowest orders of in- 

 sects. The orders described 

 below in this chapter are also Fig. 16. — The mole cricket of 



the northern United States 



of simple organization: — (Gnjiioiaipa boreaiis). 



