290 



SCIENCE- GOSSIP. 



their time in water, they not infrequently crawl up 

 and sit for considerable periods on the protruding 

 stems of aquatic plants, or take distant flight 

 in quest of further food supply. Newly con- 

 structed ponds on downs and in similar situa- 

 tions, far removed from other water, are often 

 early found to be colonised by these enterprising 

 insects. 



The commonest of our larger water-beetles, the 

 ubiquitous Dytiscus marginalis (fig. 2), so called 

 from the yellow lines with which the thorax and 

 elytra are bordered, is an insect easily examined, 

 since with a little perseverance it can usually be 

 captured in pools and ditches throughout the king- 

 dom. The young coleopterist should provide him- 

 self with a water-net made of mosquito netting or 

 similar stout material, sufficiently strong to with- 

 stand rough usage and the possible entanglement 

 amongst hedge-trimmings and other sunken debris ; 

 and thus equipped he may set to work by any road- 

 side with the full knowledge that he need, as Crabbe 

 says, "... fear no bailiff's wrath, no baron's blame ; 

 His is untax'd and undisputed game." 



By many entomologists and authors this is 

 called ' ; the Great Water -beetle " ('). but. although 

 to a certain 'extent applicable, the term does not 

 altogether commend itself as a happy one, since it 

 might easily lead one to the supposition that it was 

 our largest aquatic species, whereas that distinction 

 really belongs to Hydrophilus piceus, a beetle 

 which, although formerly included by Linnaeus in 

 the present genus (Dytiscus), is now recognised as 

 belonging to an entirely different division of the 

 Coleoptera. It measures fully one-fourth longer 

 and about a fifth broader than Dytiscus marginalis. 

 Both Linnaeus, in his " Systema Xaturae," and 

 Fabricius threw together the aquatic groups. 



All the Dytisci are carnivorous in both their 

 larval and imaginal states, and are extremely fierce 

 and voracious, preying upon young fish, newts, 

 small frogs, tadpoles, various larvae, and other 

 aquatic organisms, not even excepting individuals 

 of their own and kindred genera. They have also 

 been known to attack the giant Hydrophilus piceus 

 above alluded to, whose seemingly invulnerable 

 armour they pierce in its only vulnerable place — 

 namely, at the narrow interstice between the 

 plates of the head and thorax. The beetles prefer 

 stagnant to running water; but. owing to the 

 necessity for frequent access to the atmosphere, 

 they do not appear so partial to ponds entirely 

 covered with " weed " as are many other aquatic 

 animals. In common with others of the family, 

 however, they are fond of lying along under the 

 banks, and by quietly approaching the margin of 

 a pool one or more of the beetles may occasionally 

 be seen resting at the surface during aeration. 



The eggs of most amphibia are laid in water. 

 Those of the Dytisci, which are elongate and 



(1) Rev. w. Houghton, 51. A., " The Great Water-beetle 

 (Dytiscus marginalis") ; " Rev. J. G, Wood, etc. etc. 



cylindrical in form, instead of being laid in 

 packets, like those of the Ephemera (Houghton), 

 or dropped at random in the water (Lyonnet), as 

 with Acilius, are deposited singly in the stalks of 

 rushes, pond-weed, and other aquatic plants (-), 

 the ovipositor of the female being furnished on 

 either side with sharp instruments, for the purpose 

 of making incisions in the submerged stems to 

 receive them (Eegimbart ; Miall). Although 

 under natural conditions egg-laying, for the most 

 part, takes place during the spring and summer, 

 it may not perhaps be entirely confined to the 

 warmer months of the year. A female Dytiscus 

 punctulatus captured at West Kirby on Decem- 

 ber 19th, 1900, and kept in a cold unused upper 

 room, commenced to oviposit on the 21st of that 

 month, depositing twelve eggs in sixty hours. 

 These eggs are of an opaque white, with just the 

 faintest tinge of yellow, and, as seen under a 

 microscope, have a slightly roughened surface, 

 formed by low rounded prominences, with scattered 

 brown and black irregular markings. They are 

 slightly curved, and measure about 6J mm. in 

 length by barely 1 mm. in diameter. 



Should all go well, and the ova escape the 

 attention of various enemies, the larvae are hatched 

 in about three weeks, probably one brood only 

 being brought forth in a year. The exact period 

 would, however, be liable to be influenced by 

 temperature and other causes, cold weather tend- 

 ing to considerably retard development. The eggs 

 laid towards the end of December, above alluded 

 to, commenced to hatch out on January 31st, and 

 on the following day my wife and self were fortu- 

 nate in observing the process of liberation of the 

 young. For a short time previous to the birth of 

 a larva the egg increases slightly in size, whilst 

 the dark rings which had gradually appeared to 

 encircle it — being really the segments of the larva 

 within seen through the stretched outer covering — 

 become increasingly marked. Shortly before one 

 o'clock a slight movement of an egg under observa- 

 tion could be noticed occasionally, and at about 

 twenty minutes past the hour the larva was seen 

 to be escaping from one end of the ovum ; a pro- 

 cess which, although of a somewhat laborious 

 nature, was soon accomplished. It is then seen 

 that the escaped insect is considerably longer than 

 the egg-shell from which it has emerged. The 

 head and adjoining section, together with the last 

 segment of the body and legs, are quite white at 

 birth, all the middle portion of the larva being 

 from the first slightly tinged with brown and 

 having the rims of the segments darkly and con- 

 spicuously marked by comparison. The early life 

 of the larva is not without interest, but space pre- 



( 2) The writer in the " London Natural History of Insects," 

 vol. ii. p. 159, who states that the eggs of D. marginalis are laid 

 " in a singularly formed nidus of a silky substance which is 

 allowed by the parent to float on the surface of the water, with 

 its tapering mast, etc.," has really attributed the egg-cocoons of 

 Hydrophilus piceus to this beetle. 



