PLECOPTERA. PSOCOPTERA. 677 



Fam. 1. Perlidae. With the characters of the Order. There are a 

 couple of dozen British representatives, and as the insects are unattractive 

 and yet widely distributed it is probable that in all parts of the world 

 many remain to be described. Perla British : Pteronarcys ; Isopteryx with 

 an auditory organ on the legs : Dictyopteryx ; Isogenus British ; Nemoura. 



Order 8. PSOCOPTERA * (CORRODENTIA or PSOCIDAE). 



Minute soft-bodied insects with hair-like antennae ; wings absent 

 or present and in the latter case the fore-wings are larger than the 

 hind ones ; prothorax small and in the winged species, hidden 

 between head and mesothorax ; tarsi with two or three segments ; 

 metamorphosis slight. 



The numbers of existing and of described species is probably 

 about equal to that of the Plecoptera, but there are rather more 

 British species, some thirty or so. The antennae have from 

 eleven to twenty- five segments. The mouth parts are well 

 developed and the 1st maxilla bears a peculiar " pick." The 

 thorax looks as if it had but two segments, since in the winged 

 forms the prothorax is concealed, and in the wingless forms 

 where it is apparent the meso- and meta- thorax are fused. The 

 sterna are small and the coxae contiguous. The alimentary 

 canal is simple, there is no crop or proventriculus, only four 

 malpighian tubules, and a short in- 

 testine in Cloth-ilia. The life-history 

 of the group is not well known. The 

 development of the wings varies 

 much, but even when they are fully 

 formed there seems a curious reluc- 

 tance to use them ; still at certain FIG. 423. PSOCUS fasdatus, England 



(after M'Lachlan). 



times clouds of small Psocids float on 



the wing. Ocelli usually occur in the winged forms and are 

 absent in the apterous species. Probably the order comes nearer 

 to the Termitidae than to any other. 



Some Psocids have the wings covered with scales, which possess 

 colours, and form patterns of extraordinary beauty and com- 

 plexity, rivalling those of Microlepidoptera. Enderlein, who 

 has devoted much study to the group and described many of 

 these scaly-winged forms, f unites the Psocids with the Embiidae, 



* M'Lachlan, Ent. Mon. Mag., iii. 1867, p. 177. Derham, Phil. Trans. 

 xxii, 1701, and xxiv, 1704. Enderlein, Zool. Anz., xxvi, 1903, p. 423. 

 f Spolia Zeylanica, vol. iv, Dec. 190(>, pp. 39-1 2 and plates. 



