28 1 



to be, as a whole and comparatively speaking, feeble ; although 

 doubtless capable at times of extended flight, particularly when 

 aided or compelled by the wind, as a rule they seem to prefer, by 

 day, at least, to trust to their legs for locomotion, as indeed is 

 implied by their popular name. "On certain occasions, how- 

 ever ,they have been seen flying in one direction in the day time 

 in such numbers as to form a migratory swarm, quite like that 

 which occurs in the case of certain locusts, dragon-flies, butter- 

 flies and other insects." "it would appear from o'bservations 

 made, that these leaf-ho])i)cr migrations are largely due to the 

 fact that the food supply in the place whence they originate has 

 become exhausted or impoverished by the number of the in- 

 sects." (Perkins.) 



The maximum amplitude, in comparison with the size of the 

 body, is attained by the Poekillopteridae, which however, are 

 l)ut feeble flyers ; actual extent is greatest among the Fulgori- 

 dae, which likewise are not distinguished for powerful flight. In 

 many Issidae and other forms, tegmina are fully developed, but 

 are corneous and often convex, while the wings are also often 

 rudimentary. Perhaps the most remarkal>le tegmina are 

 found in. certain Derbidae, [Pliiladclphcia err) in which they are 

 very elongate and narrow. 



In this group is developetl very largely the remarkable and 

 little understood phenomenon of pterygo-polymorphism; that is 

 to say that in the same species there may be two or uiore forms 

 of tegmina and wings — sometimes a sexual di-or polymorph- 

 ism, sometimes not — from completeness to great reduction or 

 almost entire albsence. 'It is especially encountered in the Asira- 

 cidae and in the Tetigoniid Pliryiioiiiorplius and its allies; in the 

 Heteroptera it occurs extensively in the Gerridae and Nabidae. 



Among Hemipterists, the subject has been discussed at some 

 length bv [ohn Sahlberg (1871 pp. 19-2^) and O. M. Renter 



(1875). 



The North American and Australian forms have as yet been 

 too slightly investigated; nor am I aware of the exact propor- 

 tions in the palaearctic forms, but Sahliberg states that 70 out 

 of the 263 Scandinavian Leaf-hoppers (in 1870) are pterygo 

 dimorphic, that is, more than one-fourth. 



In most of the dimorphic Heteroptera (and in speaking now 

 of the "poly-" or "dimorphism." I am alluding only to the poly- 

 or dimorphism of the organs of flight) the thorax is somewhat 

 profoundly modified, by the reduction of the organs of flight, 

 owing doubtless to the degeneration of the supporting mus- 



