QUEKETT MICROSCOPICAL CLUB. 663 



pearancc or disappearance of Hydrodictyon is not understood. 

 After being very plentiful it will totally disappear perhaps for 

 several years, and then there is a sudden reappearance. These 

 outbreaks are known in some parts as the " breaking of the 

 meres." For instance. Hydrodictyon appeared formerly in 

 the lake in Kew Gardens. Mr. Burton has looked for it for 

 more than thirty years there, and only found one very small 

 specimen up to last autumn, when a tremendous outbreak 

 occurred. In less than four weeks, however, it had all disap- 

 peared. He suggested that the probable explanation was a 

 combination of several favourable circumstances which do not 

 frequently arise, possibly some special type of weather and 

 some narrow range of temperature at a particular season. 



Mr. F. J. Perks (Hon. Treasurer) read some Notes by Mr. 

 E. M. Nelson on various insect structures. The wing of the 

 Neuropteron, Agrion pulchdlum, he pointed out, is a very 

 interesting microscopical object. The membrane is double, 

 bordered by a rim edged with saw-like teeth, the surface is 

 divided by nervures which are peculiar the transverse bars, as 

 well as four longitudinal, have on one edge thorns, and on the 

 other saw-like teeth ; three other longitudinal ribs have saw- 

 teeth on one edge and fine teeth on the other, but no thorns. At 

 one part on the edge of the wing is a dark-coloured compartment 

 improperly called Ci stigma." This is really a pocket, and is 

 obviously used for producing a sound. If the border of the 

 wing is examined through a half-inch objective and a x 10 eyepiece, 

 a delicate hair can be seen between the teeth of the saw, very 

 minute, the largest found measuring 23 /x in length and 2 /x in 

 breadth. They spring out of circular rings, as do most insect 

 hairs, but not like those on the membrane of a blowfly's tongue, 

 which have no rings. Mr. Nelson points out that careful ex- 

 amination of the small hairs on the wing of a wasp will show 

 they are twisted like the tusk of a narwhal. The hairs on a 

 bee's wing are similar, but not so twisted, while they have no 

 ring. Those on the wing of a saw-fly issue from a boss. The 

 hairs on the ovipositor of Phalangia have a ringed base, and' 

 on the last two terminal stripes, where the hairs are larger, the 

 ringed boss has a circle of minute hairs. The hair itse f is 

 tubular, has a filamentous end, and at the side there is a minute 

 prong. At the end of each of the two lobes of the ovipositor 



