43 



There are mariy groups of insects, however, which exhibit no 

 striking variety in génital structure, and hère isolation would seern 

 ito be dépendent on psychological factors when geographical and 

 seasonal barriers are wanting. Thèse obscure aversions are^ less 

 easy of démonstration than the anatomical inconformities, but they 

 are none the less clearly indicated through an occasional lapse from 

 normal behavior. Interbreeding seems to be moderately fréquent 

 in some orders, since Lepidopterists now and then record the cap- 

 ture of hybrids under natural conditions, and it is not difficult lo 

 produce them artificially with certain species, but among the 

 Hemiptera interbreeding and hybrid offspi'ing are alike extrême! y 

 rare. In the Gerridae (water-striders) it is quite usual to meet 

 with several related species swarming in vast numbers on the sui' 

 face of a smali pond, as in the instance of the original discovery 

 of Gerris argenticoUis Parsh. On this occasion three closely rela- 

 ted species, margmatus Say, l-uenoi Kirkaldy and argentiaollis, 

 were very abundant in a small area, ail actively pairing, but of 

 many couples examined not one proved to consist of two species. 

 The genitalia of Gerris are not well.known in their détails but there 

 seems to be Qomparatively liltl« diversity in this respect among 

 the species and hênce we must attribute their isolation to psycho- 

 logical aversion. 



That copulation is anatomically possible between even remotely 

 related species of this group is proved by an observation made 

 during the last summer, while I was collecting on Long Island, 

 New York, with my friend, J. R. de la Torre-Bueno. On a small 

 pond we met with Gerris remigis Say in large numbers, many 

 being in copulation, I noticed one pair which seemed peculiar 

 and on capture it was seen to consist of a maie remigis and a female 

 riifoscuteUatus Latreille. This female was the sole représentative 

 of tlie species noted in the neighborhood . The connection belween 

 the genitalia of the two individuals seemed to be complète and 

 normal, and the couple finally separated with the usual reluctance, 

 only under the influence of the letha! collecting bottle. Thèse 

 forms are not only distinct species but even belong to two groups, 

 Gerris and Linmoporus, which are certainly good subgenera, if not 

 worthy of generic rank as some maintain. I had no opportunity 

 of rearing the oiïspring of this female, but the experiment, if car- 

 ried out, would probably hâve been inconclusive, as tliese insects 

 mate repeatedly and in fact the female was aiready gravid. It is 

 "quite likely that if connection with a maie of the same species had 

 not aiready occurred any eggs laid would bave proved infertile. 



This instance of miscegenation, involving an exceptional break- 

 ing down of the usual psychological barriers between the species, 



