THE GENERAL FEATURES OF WINGS 83 



In the Lepidoptera, in descriptions of which the term discal cell is most 

 often used, it is cell R + M + ist Mo that is thus designated. The forma- 

 tion of a large discal cell is due in this case to the atrophy of the base of 

 media, which results in the combination of the three cells named above into 

 one. 



In the Diptera it is cell ist Mo that is termed the discal cell; and in the 

 Trichoptera it is cell R2+3. 



The hypostigmatic space. — In the Myrmeleonidae and in some other 

 families of the Neuroptera, there is a greatly elongated cell behind the point 

 of fusion of veins Sc and Ri, this is termed by Tillyard the hypostigmatic 

 space. 



