SCIENCE-GOSSIP. 



201 



HABITS OF HYDROMETRA STAGNARUM. 

 By Gilbert J. Arrow. 



DROBABLY no specimen of this strange insect 

 has hitherto been watched during the entire 

 period of its development, and as I have been 

 fortunate enough to rear a brood of them from the 

 eggs to maturity, it may be interesting to record a 

 few observations which I have been able to make. 

 Even though these may not be of any very con- 

 spicuous scientific value, they will in all probability 

 have the merit of originality. 



For the benefit of those who are not yet 

 acquainted with the species, I may begin by stating 

 that it is a hemipterous insect belonging to the 

 surface fauna of our ponds and rivers, being chiefly 

 remarkable for its extremely attenuated form, 

 although a member of a group characterised by 

 great slenderness. The length of a full-grown 

 specimen is slightly more than half an inch, whilst 

 its greatest breadth never exceeds one twenty- 

 fourth of an inch. Of this length the head occupies 

 one third, being exceedingly narrow in the middle, 

 near which the eyes are situated in very prominent 

 hemispherical masses, and widening towards the 



Hvdrometra stagnarum. {Enlarged.) 



thorax and the extremity, where the antennae 

 originate. These, as well as the legs, are pro- 

 portionately long and slender, resembling fine 

 hairs. This attenuated structure is very beneficial 

 to the insect, as it renders it practically invisible 

 even to the keenest eyes except at very close range, 

 and therefore, in spite of its very remarkable 

 appearance, it is little known. It is a common 

 insect, nevertheless, and when carefully looked for 

 may usually be found without much difficulty close 

 to the margins of ponds or slowly-running streams. 

 When detected it is much more easily caught than 

 its more familiar and more active relations of the 

 genus Gerris. Unlike these and most other members 

 of the surface fauna, it is not gregarious, although 

 when one is found there are generally plenty more 

 in the vicinity. 



There are apparently two generations of 

 Hydivnictra in the course of the year, for I have 

 seen newly-born specimens as late as the middle of 

 August, although the brood to which I have above 

 referred was hatched on May 25 th. The eggs 

 were deposited by the parent insect in an aquarium 



about three weeks previously, and the first larva 

 reached maturity on June 27th, its development 

 thus occupying almost exactly a month. The 

 larvae left the egg in a highly developed condition 

 and underwent very little external change in their 

 progress to maturity. They were active little 

 creatures, about one twenty-fourth of an inch long, 

 of a transparent reddish-brown colour, with long 

 legs and antennae. They were very slender, 

 although not so strikingly so as the perfect insects, 

 the disproportion between length and breadth in- 

 creasing with their growth. Wings are absent in 

 all stages, so that it would be hardly possible to 

 mistake the identity of the larva. 



For some days after their birth the young 

 insects lived mostly upon terra firma, only occasion- 

 ally venturing upon a short excursion on the water 

 close to the edge. By degrees, however, they took 

 more to the water, at last living almost entirely 

 upon it. Until nearly full grown they fed entirely 

 upon the minute springtail (Smynthurus niger), 

 which was abundant on the surface of the water. 

 Probably on account of its minute size suiting 

 their capabilities, they at first refused all other 

 food, but shortly before assuming the perfect form, 

 they also took other small insects, such as aphides. 



Quite unlike Gerris, which moves very rapidly 

 over the water by long jerking strides, Hydrometra 

 progresses in a regular, deliberate manner, which 

 assists in rendering it inconspicuous. When in 

 search of food, it moves slowly over the water, its 

 long legs carrying the body high above the surface, 

 and the antennae bent downwards in front of the head, 

 so that their tips are all but in contact with the 

 water, appearing to " feel " it for the motions which 

 indicate the presence of its prey. Although the eyes 

 are very large and prominent, they seem only capable 

 of detecting the insects upon which the Hydrometra 

 feeds when at a very short distance ; for, unless 

 this is the case, the latter never moves directly 

 towards its victim, but reaches it by repeated 

 " tacks," each bringing it more in the desired 

 direction, the antennae being all the time engaged in 

 " feeling " the surface of the water. At last the 

 doomed insect is exactly between the forelegs 

 of its captor, and after a few preliminary 

 flourishes of the antennas, apparently to satisfy 

 itself that it is lit for food, the long rostrum 

 is slowly and deliberately brought out of its 

 position along the underside of the head, 

 " presented " for a moment at the victim, and then 

 with a movement of the whole body, which brings 

 the head down between the front legs, plunged into 

 it, or rather the rostrum is pressed against it, and 



