504 



INSECTA. 



inclose themselves. According to Roesel, the eggs of the DijUhcus ma-rginalis hatch ten or twelve 

 days after being deposited : at the end of four or five more, the larva is already four or five lines long, 

 and moults for the first time. The second change of skin takes place at the expiration of a similar 

 interval, and the animal is now as large again as it was before : when full grown it is two inches long. 

 In summer it has been observed to become a pupa at the end of fifteen days, and a perfect insect in 

 fifteen or twenty more days. 



This great genus is divisible as follows : — 



The majority have the antennae composed of eleven distinct joints ; the outer palpi filiform, or 

 slightly thickened at the tips, and the base of the hind-legs exposed. 



Dytiscus, has all the tarsi composed of five distinct joints ; the three basal joints of the fore-legs being: very 

 large, and forming- an oval or orbicular plate. Type, D. marginalis, Linn., a very common British species, an inch 



and a quarter long, being of a dark olive colour with a 



buff-coloured margin entirely round the thorax, and a 



line of the same colour on the outer margin of the elytra, 



which are not dilated at the sides ; those of the female 



are furrowed from the base about two thirds of the whole 



length. Fabricius says, that the species when laid upon 



its back gains its ordinary position by taking a leap. 



Esper kept a specimen of this insect for three years and 



a half in good health in a large bottle of water, feeding it 



every week and sometimes oftener with bits of raw beef 



about the size of a nut, upon which it precipitated itself 



and sucked the blood entirely from it. It was able to fast 



Fig. 53 — Dytiscus margiiisiis and its larva. for a month at a time. It killed a specimen of Hydro- 



philiis piceus, although as large again as itself, by piercing it between the head and thorax, the only part of the 



body without defence. According to Esper, it is sensible to the changes of the atmosphere, which it indicates by 



the heights at which it keeps in the bottle. 



Dytiscics Rceselii, Fab., [the type of Curtis's genus Cybister, or Tragus of Leach], is much more depressed than 

 the preceding, and has the outer margin of the thorax and elytra yellowish ; these elytra are finely striated in the 

 female ; the hind legs have the tibiae very short and broad. It is found in the neighbourhood of Paris and in Germany, 

 but is extremely rare in England. 



Dytiscus serricornis, Paykull, is very remarkable for the antennae of the male having the four terminal joints 

 forming a compressed and toothed mass, whence Dr. Leach formed it into his genus Agabm ; other characters, 

 such as the form and relative proportions of the joints of the outer maxillary palpi, have also led him to form other 

 genera, namely — Hydaticus (Dyt. Hybneri, transversalis, &c.) and Acilim (D. sulcatus\. [These various groups, 

 here reduced by Latreille to the subgenus Dytiscus, are far better marked than many of the groups adnntted amongst 

 the Carabiques possessing characters, not only in the imago, but also in the larva states, amply sufficient to warrant 

 their separation.] 



Colymbetes, Claiv., has all the tarsi distinctly 5-jointed, but the four anterior tarsi in the males are equally dilated 

 into one small oblong plate, and the antennae are at least as long as the head and thorax ; the body is perfectly 

 oval, and broader than deep, and the eyes are not exposed. Types, Dyt. fuscus, Panz., D. cinereus, Fabr., Panz., 

 &ic. [These insects are of an intermediate size between the foregoing and following species, and fonri a very exten- 

 sive group. Erichson, Eschscholtz, and Aub^, have particularly studied this group, and have proposed various 

 dismemberments from it, which have been partially adopted by more recent authors.] Some of the smaller species 

 without a visible scutellum, and with the anterior tarsi scarcely dilated in the males, compose Leach's genus 

 Laccophilus ; such are the D. hyalinus. Marsh., D. minutus, Linn., &c. 



Hygrobia, Latr. (Hydrachna, Fabr., Pcelobius, Schonh.), have the four anterior tarsi in the males also equally 

 dilated into a small oblong plate, but the antennae are shorter than the head and thorax ; the body is ovoid, very 

 thick in the middle, and the eyes very prominent. Type, H. Hermamii, Latr., [a common British species]. 



Hydroporus, Clairv., has the four anterior tarsi spongy beneath in both sexes, with only four distinct joints, the 

 ordinary fourth joint being obsolete or very small, and hidden, as well as the base of the following, in a deep notch 

 of the third. The scutellum is not visible. The body is oval. Types, Dytincus inequalis, picipes, &c. 



Hyp/iydrus, Latr., consists of such species of the latter as have the body nearly globular, and the last joint of the 

 four anterior tarsi is very small, and scarcely extending beyond the preceding. H. gibba, ovalis, scripta, Fabr. 



Noferus, Clairv., differs from all the preceding by having the antenna; dilated in the middle, and the last joint of 

 the labial palpi is notched, so as to appear forked. Dytiscus crassicornis, Fabr. 



Halijilus, Latr., (Hoplitus, Clairv., Cnemidotus, Illig.) forms a distinct section having only two distiuct joints in the 

 antenna; ; the palpi terminated by a small joint pointed at the tip, and the base of the hind legs covered by a large 

 plate. Types, Dytiscus fidvus, impressus, obliquus, and many other species of very small size. 



[The family Dyticidfe of English authors has been investigated by several recent authors, especially 

 by Leach, in the Zool. MiscelL, vol. iii. ; Erichson, in his Genera Dtjticeorum, and* Kafer dcr 

 Mark Brandenburg ; Laporte in the Etudes Entumoloyiques ; Say in the American Phil. Trans. 



