HARD WICKE'S SCIENCE- G OSS IP. 



49 



THE BRITISH PERLIDjE OR STONE-FLIES. 



By W. H. NUNNEV. 



[Continued Jrom p. 39.] 



T will be seen from 

 the table (p. 38) 

 that the British 

 Perlidae may pri- 

 marily be divided 

 into three sub- 

 families : — Per- 

 lin^E, including 

 those genera in 

 which the species 

 have setaceous 

 palpi and long 

 tail-bristles — Cap- 

 NIINvE, the species 

 of which possess 

 filiform palpi and 

 long tail-bristles — 

 — and lastly Ne- 

 MOURIN/E, includ- 

 ing all species with 

 filiform palpi and 

 tail-bristles which are merely rudimentary or are 

 entirely wanting. Some systematists may possibly 

 raise objections to such a division of the family, but I 

 am convinced that the distinctions between the groups 

 I have named are something more than mere generic 

 ones. Not, however, having space at my disposal in 

 which to enter at length into my reasons for such 

 belief, I will now proceed to give notes relating to 

 the various species mentioned in the synopsis. 



Dictyopteryx microcephala, Pictet. This insect 

 appears to be widely distributed. It is common in 

 the south of England, and in Ireland, and is found 

 somewhat sparsely in Scotland. It frequents the 

 borders of streams from early spring until autumn. 



The wing venation is not constant, the cross-veins 

 beneath the costa especially varying in number in 

 individuals. In a specimen in my own collection, 

 No. 327. — March 1892. 



the left upper wing has only five cross-veins, whilst 

 the right upper wing has eight. In Pictet's drawing 

 of this species, in Vol. 26 of the " Annales des 

 Sciences Naturelles," he shows seven sub-costal 

 cross-veins. The size also varies slightly, the average 

 length of the body with the wings closed being nine 

 lines, and with the wings expanded, one inch five 

 lines. The wings are considerably shorter in the 

 male than in the female. 



Dictyopteryx rectangula, Pictet. This species has 

 the wings somewhat broader and of a darker tint, 

 with very dark nervures. It is rather common, and 

 widely distributed in the south, along the banks of 

 streams in June. Its length is nine lines, wing 

 expanse one inch four lines. It differs from micro- 

 cephala in its smaller size, the greater width of the 

 hinder margin of the prothorax, and the slightly 

 different reticulation of the sub-marginal region ;in 

 microcephala the cellules are small, irregular, hexa- 

 gonal or pentagonal, whereas in the present species 

 they are rectangular. 



Isogenus nubecula, Newman. This insect, the 

 only European species of the genus, is a connecting 

 link between Dictyopteryx and Perla, and Perla and 

 Chloroperla. It is found in the neighbourhood of 

 running water, and is apparently widely distributed, 

 except towards the north, where it is somewhat 

 scarce. This species is easily distinguished from 

 allied species of other genera by its wings having 

 a small oval dark-brown spot on the costal margin, 

 about two-thirds of the distance from the base to the 

 tip. Length nine lines ; wing expanse fifteen 

 lines. The male is less in size than the female. It 

 appears in April. 



Laboulbene states that specimens of this species, 

 when laid upon their backs, remain perfectly motion- 

 less, excreting at the same time a yellowish liquid at 

 the joints of the legs. 



D 



