SCIENCE- GOSSIP. 



169 



completed floating structure slightly attached to 

 a bit of weed (-). 



A. G. Laker also has given particulars of the 

 same beetle's cocoon, of which he found a number 

 of specimens in a pond near London. They varied 

 considerably in size and shape, but averaged about 

 \\\ lines long by lOf lines broad, the height to 

 the tip of the spike being about 17 lines. The 

 walls were composed of a substance very like 

 paper ; but the part immediately below the spike 

 was of a loose silky material, which, though this 

 part was submerged, was not impervious to water. 

 It was by breaking through this material that the 

 young larvae ultimately escaped. The interior of 

 the cocoon contained similar loose material, as 

 well as the eggs, of which there were fifty to sixty. 

 The cocoons were remarkably buoyant ; but they 

 usually floated attached to confervae, to long grass 

 growing in the water, or to the under-side of 

 floating leaves. The spike was of a substance 

 somewhat thicker and stronger than the rest of 

 the cocoon, and was hollow throughout the greater 

 part of its length, except that it was crossed and 

 recrossed inside with a dark thread-like substance. 

 The observer compares it to a horn stuffed with 

 tow ( 3 ). 



There are in various parts of the world a number 

 of other species of HydropMlm, but the writer has 

 not seen observations on their spinning-work, ex- 

 cept in the case of the great American Hydro- 

 phi I us triangularis. I am not aware that the 

 spinning process has been watched in this species ; 

 but the cocoon itself has been carefully described. 

 Garman, who obtained half-a-dozen cocoons from 

 the surface of a small pool, says that the outside 

 is smooth, of a light brown colour, and much 

 resembles the egg-cocoon of the common black 

 and yellow Epeirid spider of North America. 

 Viewed from above or below, the outline is 

 circular, with a diameter of about 20 mm. The 

 spike, unlike the rest of the cocoon, is of a horny 

 nature. Its base expands into a hatchet-shaped 

 plate, the spike being formed, in fact, by the 

 narrowing and folding back of this plate until its 

 edges almost meet. The process is thus a partial 

 tube, which, projecting above the water, appears to 

 facilitate the entrance of air into the cocoon. 

 Below the expanded base of the spike is a narrow 

 opening through which water passes, and which 

 finally permits the escape of the larvae. The eggs, 

 numbering in one case 107, form a discoid mass 

 surrounded by a loosely-woven silken coat and 

 suspended from the roof of the cocoon. Below, at 

 the sides, and behind the egg-mass, is a space to 

 which water has access ; but the silken material 

 above and in front of the eggs is disposed so as to 

 form large cells, and these connect with the air- 

 admitting spike. The chief object of the cocoon, 



(2) Fullagar, " Hardwicke's Science-Gossip," xv. (1870\ pp. 

 132, 133 



(3) Laker, " Entomologist," xiv. (1881), pp. 82-81. 



according to this observer, is to ensure a supply ( f 

 air for its contents ; and this is done, be -ays. " by 

 excluding water from, and admitting air into, the 

 upper part of the case" ( 4 ). Riley has written on 

 the same subject, confirming ( rarman's descripl ion. 

 He observes that the cocoon differs from that of 

 HydropMlm piceus "according to the descrip- 

 tions of this last;" but, when the cocoons them- 

 selves are compared, it is possible that they will 

 be found much alike. Riley gives figures and an 

 ideal section of 'he cocoon of HydropMlm 

 triangularis. It consists, he says, of three distinct 

 parts : — 



"There is first what may be called the floater, 

 which itself is composed of two parLs, viz.: — 

 (1) a hard spatulate piece of compact brown silk, 

 smooth externally, and with the two edges of the 

 tapering end curled inside and welded at tip, so as 

 to form a stout point [the spike], and (2) a some- 

 what cuneiform air-chamber. There is, second, 

 the egg-case proper, and, third, the outer bag or 

 covering. The air-chamber has an external slightly 

 bulging covering of the same character as the 

 outer bag, of which it forms a part, but of some- 

 what darker silk, while the inside consists of loose 

 brown silk, forming large cells and connecting 

 with the spine, the hollow parts of which are in 

 fact filled more or less compactly with these 

 silken fibres. The egg-case proper, which is of a 

 white, rather flimsy or paper-like silk, is partially 

 suspended posteriorly from the roof of the outer 

 bag by white loose silk, but is principally attached 

 to the interior side of the air-chamber. - ' 



Between the egg-case proper and the outer 

 covering is the space to which water is admitted ; 

 and the larvae, Riley states, breaking through the 

 egg-case proper, remain for a day or two within 

 this space, issuing ultimately through the rent at 

 the foot of the spike. When the cocoon is floating, 

 the spike is directed upward, most of the " floater " 

 being out of the water. The eggs, though bathed 

 in water, are thus freely aerated, the function of 

 the cocoon being, no doubt, to secure a supply of 

 air, and at the same time to protect the eg^s and 

 newly-hatched young from numerous enemies ( 3 ). 

 The contrivance is clearly a complicated ami 

 curious one ; and it is obvious that the spinning 

 instincts of these beetles are of no mean order. 



Cocoon-spinning is not confined to HydropMlm, 

 naturalists having long known it to obtain in the 

 allied Hydroclw/ris, as well as in many of the 

 smaller Hydrophilid-beetles, of which, perhaps, 

 our little Hydrobius fuscipes is the best known. 

 Among recent writers, Perkins notes the finding of 

 nests of llydroi-haris oa/raboides. He states that 

 the insect, rolling up a leaf, lines it with a thick 

 cottony web-like substance ( ,; ). Laker, who writes 

 of the egg-cocoons of Hydrobius fusr'qics. compares 

 them to the relatively gigantic structures of 

 HydropMlm ; but they are flattened on one side, 

 (i) Garman, "American Naturalist," xv. (1881), pp. 600, 

 601. 



(5) Riley, " American Naturalist," xv. (1881). pp. 814-817. 



(6) Perkins, "Entomologist," xii. (1879J, pp. 214-216. 



G 3 



