A GUIDE TO THE STUDY OF BRITISH WATERBUGS. 7 



In Aphelocheirus, Plea, and Notonecta the structure is fairly 

 simple, and two claws are present in each tarsus, inserted at the 

 apex. In Ilyocoris the femur is enormously dilated, forming an 

 irregularly equilateral triangle ; the tibia is slender, and fits at 

 rest into a groove (sulcus) on the ventral surface of the femur ; 

 the tarsus is minute, composed of only one segment without 

 claws. In Nepa the tibia is much longer, and fits into a 

 similarly placed groove in the femur, the latter being slenderer 

 and longer than in Ilyocoris. In the former there is also only 

 one segment in the tarsus, which is minutely falcate at the apex, 

 but there are no claws. In Ranatra the coxae are very long, and 

 were formerly often mistaken for the femora, which are rather 

 longer and [in the British species R. linearis (Linn.)] are armed, 

 about a third of the entire length from the apex, with a stout 

 triangular spur ; the tibia and tarsus are short, folding in close 

 to the femur and locked by the femoral spur ; there is only one 

 segment in the tarsus, which is not provided with claws. The 

 anterior legs in Corixa and Micronecta are exceedingly curiously 

 formed, being more or less spoon-shaped. They will be discussed 

 in detail later. 



The posterior legs are those principally employed for loco- 

 motion. In Hydrometra and Gerris they are enormously long 

 and very slender ; in the other genera they are usually of 

 medium length and thickness, more or less ambulatory. In 

 Corixa, Micronecta, and especially Notonecta they have well- 

 developed natatory powers. Claws are present in the Gymno- 

 cerata, Nepidae, Naucoridae, and Plea. The intermediate pair of 

 limbs appear to be partly employed for progression, though in 

 Corixa and Notonecta they certainly act as balancers. In the 

 Gymnocerata, Nepidae, Naucoridae, and Plea they are not very 

 dissimilar to the posterior pair ; in the others they are long and 

 slender. Claws are present in all. 



I now purpose to deal systematically with the aquatic forms, 

 discussing any special points of structural interest, and men- 

 tioning such of the habits, &c, as have been recorded, confirming 

 or extending these as far as possible from my own observations. 



Hebrus. This genus, the systematic position of which is still 

 doubtful, is readily distinguished from Hydroessa {= Microvelia), 

 which is superficially similar, by the five-segmented antennae,* 

 and by the shape of the pronotum. The two British species may 

 be found amongst Lemna and Sphagnum in marshy ground, but, 

 as far as I am aware, no detailed observations of their habits 

 have been published. H. pusillus, Fallen, which seems to be 

 always macropterous, is figured by Douglas and Scott and by 



* The presence of five distinct antennal segments is not a family 

 character, as the extra-British genus Merragata, F. B. White, possesses 

 only four. The extra two or three segments in Hehrus, enumerated by 

 certain authors, are merely nodes. 



