112 THE ENTOMOLOGIST. 



Two brief papers * have been published by Arrow and 

 Palumba upon the habits, etc., of the species before us. 



Arrow has observed that some ova hatched in three weeks 

 (May 25th), the first nymph maturing June 27th. The young 

 nymphs live on terra fir ma, occasionally making excursions on 

 to the water, but keeping near the edge. Until almost full- 

 grown, their food consists of the Black Water Springtail (Smyn- 

 thurus niger), which seems to form the staple nourishment of 

 most small aquatic insects. 



H. stagnorum seems well distributed in England, and has 

 been recorded from Moray and Perthshire, in Scotland, and from 

 Armagh in Ireland. The head is figured on Plate i., fig. 10, and 

 the right anterior tarsus on fig. 5. 



The semi-aquatic forms which, up to the present, we have 

 considered — viz. Hebrus, Aepophilus, Mesovelia, and Hydrometra 

 — form a very heterogeneous group ; they have, however, several 

 features in common, which they do not share with the succeeding 

 genera, viz. : Velia, Hydrocssa, and Gerris. The former move 

 opposite legs alternately, in the act of progression ; the latter 

 move them together. The claws are inserted at the apex of each 

 tarsus in the former, considerably below the apex in the latter. 

 (Plate i., figs. 4 and 5.) The constituents of the former group 

 are but little related to one another and perhaps still less to the 

 latter group, but with the scanty knowledge we possess at present 

 of their metamorphoses, habits, and anatomy, it seems useless 

 speculating upon their affinities, and convenient to place them 

 all in one family, Gerrid^e (= Hydrometridce auctt. + Hebrides 

 auctt.). 



The following table will readily separate them : — 



1. Claws inserted at the apex of each tarsus (fig. 5); 



opposite legs moved alternately in progression 2 



— Claws inserted below the apex of each tarsus 



(fig. 4) ; opposite legs moved together in pro- 

 gression 5 



2. Coxae practically contiguous 4 



— Coxae widely separated 3 



3. Antenna? with five distinct segments ; head short, 



ocelli present ; tarsi with two segments. 

 Found in Lemna, Sphagnum, etc Hebrus. 



— Antennae with four segments ; head very elongate 



(figs. 10 and 5), ocelli absent ; tarsi with 

 three segments. Found near the edges of 

 ditches and streams , Hydrometra. 



* Arrow, " Habits of Hydrometra stagnarum" (sic), ' Science 

 Gossip,' 1895, ii. pp. 201-3. Palumba, " Gli amori del V Hydrometra stag- 

 norum" ' Bivist. Ital. Sc. Nat.' 1891, xi. p. 1. There is, apparently, also 

 some information anent the habits, etc., in Griffini's ' Gli Insetti Acquaioli ' 

 (Turin) 1894, which I have not seen. 



