N0M5MBER 1, 1889.] 



KNOWLEDGE. 



^^^ AN ILLUSTRATED "^^ 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: NOVEMBER 1, 1889. 



CONTENTS. 



The Common Cockroach. — IV. By E. A. Bdtler 1 



The Fish Lizards of the Secondary Rocks. By R. 



Lydekkkr. B.A., C.Tntab. ... .. ... ... ... 3 



The Ethnological Significance of the Beech. By Canon 



Isaac Taylor, Litt.D., LL.D 5 



Drawings of the Milky Way. By A. C. R 6 



Some Properties of Numbers. By Robt. W. D. Christie 7 



Colour Blindness. By Un hard Beynon 8 



On Large Telescopes. By A. C. Rantakd 9 



Barnacles. By S. Heywood Seville 10 



Notices of Books 12 



Letters: — \V. Stanifouth, H. Combes ... ... ... ... 16 



The Face of the Sky for November. Bv Herbert 



Sahi.er, F.R.A.S. " 17 



Whist Column. By W. Montagu Gattie 18 



Chess Column. By I. Gunsberg 19 



THE COMMON COCKROACH.-IV: 



By E. a. Butler. 



IN order to complete the sketch ah'eady giveu of the 

 digestive system, we have yet to notice some im- 

 portant accessory organs, the saHvary glands, which 

 in the cockroach are enormously developed. On 

 (litrang the body, they may be seen lying along just 



outside the walls of the crop ui its anterior part. Each 



consists of a pair of white glands (Fig. 9) and a very thin- 

 walled, elongated bag or receptacle, 

 quite distinct from these ; each gland, 

 though apparently compact, really 

 consists of a flattened tree-like struc- 

 ture, the foliage portions of which 

 contam the cells which secrete the 

 saliva, while the branches are minute 

 tubes which convey it, when secreted, 

 from the gland. A narrow tube, or 

 "duct," formed by the union of the 

 above branches, receives the salivary 

 Huid from each gland, but the two 

 ducts of each side soon unite into a 

 single one, and the two thus formed 

 again coalesce and form one main 

 centnil tube lying under the resopha- 

 gus.' > Similarly the ducts of the 

 receptacles unite in a single central 

 caniil, which receives, a little beyond 

 this junction, th(> common duct of 

 the glands, and so finally only one 

 tube remains as the representative of 

 the original six, and this opens into 



the mouth behind the tongue. 



Like otlier terrestiial insects, a cockroach breathes by 



taking in air, not through its mouth, but at certain 



Fig. 9. — Salivary 

 Glands oif Cockroacu. 

 !/, glands ; r, recep- 

 tiiclea : ly, common duct 

 of gliiiids ; rr, conmHin 

 duel t.f rfc(.|.t.icles. 



openings in the sides, called spiracles, or utii/mata. There 

 are ten of these on each side, eight pahs being situated in 

 the abdomen and the other two in the thorax. Those 

 belonging to the abdomen are not very easy to detect, as 

 they are small and all but one pair obscurely situated. 

 The chitinous integument which bounds and determines 

 the form of each segment of the abdomen is not a complete 

 ring roimd the body, but consists of two distinct parts, a 

 band across the back, called the teiyum, and another imder- 

 neath, the Mciiuiw ; these are imited towards their edges 

 by a menibranous jmrction, and it is in this at the junction 

 of the segments that the stigmata he, concealed by the 

 overlapping edges of the terga and sterna. Each is an 

 oval aperture situated on the summit of a small conical 

 eminence (Fig. 10), and capable of bemg closed bv an 

 internal valve, whereby dust and 

 other foreign matters are excluded. 

 A large tracheal tube proceeds from 

 each, and very soon begins to sub- 

 divide into smaller ones, the ulti- 

 mate ramifications of which pass to 

 the remotest parts of the body, and 

 even into the jaws, wings, legs, and 

 antenna?. By means of this system 

 of tubes air is conveyed to all parts ^ Fig. K 

 of the organism, so that the blood is ^p'kacle 

 aerated, not, as in most kinds of ^°'^*'"- 

 animals, by being brought from the body at large and 

 collected in some special organ, such as a hmg or a gill, 

 there to come into contact with the air. but by ha%"ing the 

 air conveyed to it in all parts of the body at once. 



The introduction and expulsion of air is, of course, 

 accompanied by movements of the body walls, but these 

 are not very easy to see as they are but shght in amoimt. 

 Plateau succeeded in demonstrating their character and 

 extent by the ingenious method of projecting the form of 

 the body of the li\ing insect on a screen by means of 

 the lantern, and then tracmg its outline during inspiration 

 and expiration respectively. In general, an insect at rest 

 performs its respiratory movements with the hinder part 

 of its body, in other words, it pants with its (dnhmien, the 

 movements consisting of an alternate contraction and 

 recovery of shape of that region. Amongst British insects 

 there is perhaps no species in which it is easier to watch 

 these movements than the great green grasshopper, a large 

 locust-like insect found not unfi-equently in some parts of 

 the country. By the contraction of certain abdominal 

 muscles, the upper and lower walls of the abdomen are 

 drawn together to the extent, in the cockroach, of cue- 

 eighth of the entire depth of the body, and a compression 

 from side to side takes place at the same time ; the tracheal 

 tubes are thus compressed, and air is forced out at the 

 stigmata ; on the relaxation of the muscles, the elasticity 

 of the tracheal tubes themselves, resulting from the coiled 

 spiral thread in their interior, then restores the body to its 

 normal form, while air in consequence enters at the stig- 

 mata. In the cockroach, the thoracic segments have 

 sufficient mobility to permit of their taking some part in 

 the movements of respiration. e\en when the insect is at 

 rest, in which respect it dift'ers from most other bisects. 

 In order, therefore, clearly to realise how a coekroacli 

 breathes, we have to bear in mind that, concurrently with 

 the rise and fall of the body walls, ten little jets of air 

 alternately enter at and issue • from as many openings in 

 the insect's sides, the outward-tending jets of course carry- 

 ing with them the carbonic acid and water vapour produced 

 as the insect discharges its vital functions. 



But the respiratory movements above described can 

 scarcely be regarded as providing a complete explanation 



