io6 



HARDWICKE'S SCIENCE-GOSSIP. 



Lud seems to have found it easier to flow northward 

 over what is now Hubbard's Hills, than to keep to 

 the direction of the older valley, and it has since cut its 

 way deep down into the hill as already shown. After 

 passing through the hills, it joins another stream 

 which has behaved in a precisely identical manner. 

 This other stream, rising at Welton village, pursues 

 an ancient valley in the old chalk, now occupied by 

 the drift for about a mile, and then suddenly turns 

 southward and runs through a ravine known as 

 Welton Vale, a place of very romantic beauty. 

 Where it turns into the vale it leaves what -was, 

 clearly enough, its ancient bed, for the old depression 

 is very palpable round by the front of Elkington Hall, 

 and down to a lower portion of the present stream near 

 to where it joins the brook from Hubbard's Hills. 

 That old depression is still filled with boulder-clay, 

 and along the greater part of its distance is occupied 

 by another and smaller stream. Near Swaby is 

 another example of these post-glacial ravines in the 

 chalk, and at Ilatcliffe there are four of them, one of 

 which has since been deserted by the brook which 

 excavated it. 



W. Mawer. 



THE OUTER SKELETON ;0F THE COCK- 

 ROACH.* 



By Professor L. C. Miall and Alfred Denny. 



WHEN the skin of an insect is boiled succes- 

 sively in acids, alkalies, alcohol, and ether, 

 an insoluble residue known as Chitin (Cg H15 NOg) 

 is obtained. It may be recognized and sufficiently 

 separated by its resistance to boiling liquor potassn;. 

 Chitin forms less than one-half by weight of the 

 integument, but it is so coherent and uniformly 

 distributed, that when isolated by chemical reagents, 

 and even when cautiously calcined, it retains its 

 original organized form. The colour which it fre- 

 quently exhibits is not due to any essential in- 

 gredient ; it may be diminished or even destroyed by 

 various bleaching processes. The colouring-matter 

 of the chitin of the cockroach, which is amber-yellow 

 in thin sheets and blackish-brown in dense masses, 

 is particularly stable and difficult of removal. Its 

 composition does not appear to have been ascer- 

 tained ; it is white when first secreted, but darkens 

 on exposure to air. Fresh-moulted cockroaches are 

 white, but gradually darken in three or four hours. 

 Lownet observes that in the blow-fly the pigment is 

 " first to be met with in the fat-bodies of the larvce. 

 These are perfectly white, but when cut from the 

 larva, and exposed to the air, they rapidly assume 



• For "The Natural History of the Cockroaches," see this 

 Journal, March, 1884. 

 t "Anatomy of the Blow-fly," p. 11. 



an inky blackness. . . . When the perfect insect 

 emerges from the pupa, and respiration again com- 

 mences, the integument is nearly white, or a faint 

 ashy colour prevails. This soon gives place to the 

 characteristic blue or violet tint, first immediately 

 around those portions most largely supplied with air 

 vessels." Professor IMoseley* tells us that, thinking 

 it just within the limits of possibility that the browa 

 coloration of the cockroach might be due to the 

 presence of silver, he analysed one pound weight of 

 Blatta. He found no silver, but plenty of iron, and 

 a remarkable quantity of manganese. That light has- 

 some action upon the colouring matter seems to be 

 indicated by the fact that in a newly-moulted cock- 

 roach the dorsal surface darkens first. 



The chitinous exoskeleton is rather an exudation 

 than a true tissue. It is not made up of cells, but of 

 many superposed laminse, secreted by an underlying 

 epithelium, or " chitinogenous layer." This consists- 

 of a single layerl of rounded, nucleated cells, rest- 

 ing upon a basement membrane. A cross-section 

 of the chitinous layer, or "cuticle," examined with a 

 high power, shows extremely close and fine lines 

 perpendicular to the laminae. Here and there an 

 unusually long, flask-shaped, epithelial cell projects 

 through the cuticle, and forms for itself an elon- 

 gate chitinous sheath, commonly articulated at the 

 base ; such hollow sheaths form the hairs or setaa 

 of insects — structures quite different histologically 

 from the hairs of vertebrates. 



Like other Arthropoda, insects shed their chitinous- 

 cuticle from time to time. A new cuticle, at first 

 soft and colourless, is previously secreted, and from 

 it the old one gradually becomes detached. The 

 seta; probably serve the same purpose as the " casting- 

 hairs " described by Braun in the cray-fish, and by 

 Cartier in certain reptiles,! that is, they mechanically 

 loosen the old skin by pushing beneath it. The 

 integument about to be shed splits along the back of 

 the cockroach, from the head to the end of the- 

 thorax, and the animal draws its limbs out of their 

 discarded sheaths with much eflbrt. Nearly at the 

 same time the chitinous lining, which occupies a. 

 great part of the alimentary canal, and of the tracheal 

 tubes, is cast. 



The head of the cockroach, as seen from the front, 

 is pear-shaped, having a semi-circular outline above, 

 and narrowing downwards. A side-view shows that 

 the front and back are flattish, while the top and 

 sides are regularly rounded. In the living animal 

 the face is usually almost vertical, but it can be tilted 

 till the lower end projects considerably forward. 

 The mouth, surrounded by gnathites or foot-jaws, 

 opens below. On the hinder surface is the occipital 

 foramen, by which the head communicates with the 



• " Q. J. Micr. Sci.," 1871, p. 394- 



+ A condensed and popular account of these researches will 

 be found in Semper's " Animal Life," p. 20. 



