KNOWLEDGE OF THE HYDRACHNIDAE. 28? 



the transformation by ecdysis from one to the other having 

 been frequently witnessed. The most important structural 

 modification connected with this metamorphosis is manifested in 

 the genital plates, which become transformed from the basally 

 divaricant ovate plates each bearing two acetabula only of the^ 

 nymph to the elliptical form with three acetabula on each side- 

 characteristic of the adult state. It is interesting to observe 

 that the genus Atax proper is distinguished also in its nymph 

 condition by a corresponding number of genital acetabula, w^hile 

 an identical structure and arrangement is diagnostic of the 

 adult state in the genus Atractides. In addition to Limnesia 

 histrionica, the writer has taken in the neighbourhood of London 

 the L. tiyrina of C. L. Koch and a third form apparently identical 

 with the L. undulata of the same authority. Koch in his 

 Arachnidensystems refers no less than twenty species to this 

 genus, but there can be no doubt that many of these, as is the 

 case with L. cjjanipes, are merely nymph or immature phases of 

 other forms. In addition to the distribution of the genital plates 

 in the nymph of L. histrionica, here identified with Koch's above- 

 named type, it is found that the characteristic swimming-fascicles 

 of the adult are in a very rudimentary state of development, 

 being represented indeed by only a few somewhat longer isolated 

 hairs developed upon the two posterior pairs. A similar 

 feature being distinctive of Koch's L. viteUina, L. fenestrata, 

 L. liiodesta, L. sacra, L. alhella, and L. minutisshna, it seems 

 highly probable that these all belong to some nymph category. 

 His Z. longipalpis, conspicuous for the abnormal length of its 

 legs and palpi, is probably an example of a nymph recently 

 exuviated to the adult form of probably such a type as L. tiyrina 

 or L. hieroghjphka. L. histrionica, immediately after the ecdysis, 

 exhibits a corresponding long-limbed aspect. The remaining 

 species figured and described by Koch w4iich have apparently a 

 sound claim for specific recognition are L. maculata, L. ohlonga^ 

 L. 2'>hoeniceay L. undulata, L. venustata. His L. rntilas appears 

 to the writer to be a variety only of the very variably coloured 

 L. histrionica; his L. attalica identical with L. cyanipes, and 

 therefore the nymph only of the last-named species; while Koch's 

 L. ajfints probably occupies a similar position with relation to 

 L. maculata. 



The species added to this genus by recent investigators include 



