THE LIFE-HISTORY OF FRESH-WATER MITES. 363 



choose for their hosts members of the Coleoptera and Hemiptera, 

 and the same species do not always confine their attentions to the 

 same host, for I have found Hydrachna globosa parasitic upon 

 several different insects. Plate 27. a, is Ranatra linearis, Linn., 

 which 1 found in Epping Forest with a great many more lodgers 

 than I have drawn ; but I was able to keep this specimen alive 

 long enough for some to break away. They proved to be Hydrachna 

 globosa. At b is a figure of one of the parasites. Fig. c shows 

 Corixa geoffroyi, another favourite host, and Fig. e shows the 

 way in which the parasites were attached. Fig. d is another 

 Corixa, whose specific name is unknown to me. It shows the 

 larval form of a mite parasitic under the wang-cases. I found 

 another specimen with three of these large red parasites in a 

 similar position. 



Plate 28, a, is Xotonecta glavca, very commonly found with 

 the legs covered with the red larval parasites, b showing the 

 hind leg of one so infested. When these creatures leave the 

 egg they have the appearance of the larva drawn in Fig. c. 

 They attach themselves as soon as possible to their unfortunate 

 hosts, and fix their mouth-organs firmly into the insect, growing as 

 fast as the nourishment they get will allow. But they onlv grow 

 in the soft-bodied part, the dorsal plate, the epimera, and the legs 

 remaining the same size as at first (see Figs, d and e). In most 

 cases, after a time, the legs fall away, being of no further use. 

 Later on the nymph can be plainly seen inside the semi-transjDarent 

 envelope (Fig. f), and in the spring the envelope breaks (Fig. g) 

 and the mite escapes as a free swimmer with eight legs, and is 

 known as the nymph. The case from which the mite has 

 escaped remains attached to the insect, where, as far as we can 

 judge, it had been attached for twelve months, from spring until 

 spring comes round again. Fig. h is Bytiscus marginalis, showing 

 a number of parasites on the ventral surface. This is not the 

 only species of Dytiscus I have found with the Hydrachna larvae 

 attached. They are also found on D. dimidiatus, D. punctidatus, 

 and D. circumcinctus, and thanks to the kindness of Mr. Browne, of 

 the Sutton Broad Laboratory, I was able to secure specimens show- 

 ing the larval attachments. They hold on with great tenacity, and 

 I have often held one of the larvae with the forceps and gently 

 swung the beetle about without causing its separation. A writer 

 the other day in one of the country papers was speaking of 



