KNOWLEDGE 



[November 1, 1889. 



of the mechanism of breathing. For they would simply 

 have the effect of- renewing tliat portion of the contents 

 of the tracheal tubes which is in the parts nearest the 

 spiracles, and the air in the mmute and reinote subdi\'i- 

 sions of the trachea; would have no chance of being 

 expelled, and would simply oscillate up and down the tubes 

 and never reach the outer air at all. How this difficulty 

 is met is not at present altogether clear, but it seems 

 certain that we must look to the principle of gaseous diffu- 

 sion as at any rate aiding in producing the required result 

 of the penetration of oxygen to the remotest parts of the 

 system, and the corresponding outward passage of the car- 

 bonic acid formed. The rate of breathing depends upon a 

 variety of circumstances. It is quickened by whatever 

 increases the general acti\'ities of the insect ; thus, a swiftly 

 rmming cockroach breathes more quickly than one at rest, 

 and, again, a well-fed individual is naturally more vigorous 

 and inclined for exertion than a lean and emaciated one, 

 and its respiration becomes in consequence more rapid. 

 Cold, on the other hand, has a benumbing effect, and the 

 rate of breathing therefore falls with the temperature. 



The extreme perfection of the respiratory system is 

 closely connected with and correlated to a very rudimen- 

 tary condition in the circulatory apparatus. As the air is 

 conveyed to every part of the body, and oxygenation can 

 take place anywhere, there would clearly be no object m 

 having any special apparatus for the collection and 

 guidance of the blood. A cockroach has indeed, as aheady 

 mentioned, a heart, but beyond this it can scarcely be said 

 to possess any circulatory system. Anyone who brings to 

 the examination of a creature such as this the popular 

 conception of what a heart is like, will certainly fail to find 

 anything which bears the remotest resemblance to such 

 conception. There is, in fact, no compact, chambered, 

 fleshy, conical body such as we are familiar with in 

 vertebrate anatomy ; the " heart," so called from its 

 function, not its form, is merely an inconspicuous, elongated 

 soft tube, with sundry openings m its sides through wliich 

 blood enters it from the body at large. Nor is its position 

 such as might have been anticipated ; we must look for it, 

 not towards that side of the body which faces the gi'ound, 

 but on that which is uppermost, for it lies along the whole 

 length of the back, just beneath the skin, in the middle 

 Ime. Nor, again, does this rudimentary heart communicate 

 with any system of blood-vessels for conducting the blood on 

 its tour romid the body ; for the blood, on being expelled 

 from the orifice at the extremity of the tube, is simply 

 passed on through the various interstices between the 

 different viscera, until it ultimately finds itself back again 

 at the place it started from. Hence it is manifest that 

 every movement of the body wluch in any way disturbs the 

 relative position of the internal organs will assist, to some 

 extent, in urging the blood along its course. Nor, finally, 

 is the blood itself exactly what its name might suggest ; if 

 a cockroach is wounded, blood will, of course, issue from 

 the wound, but as it is only a colourless hquid, a little 

 stretch of the imagination is required to reahse that it is 

 the true nutrient tluid of the body. 



By the dissection which removes the digestive tract, the 

 main part of the nervous system is laid bare. It is con- 

 structed similarly to that of the earthworm, and consists of a 

 chain of nervous centres or ganglia, m pairs, comiected with 

 one another by nervous threads (Fig. 11), and extending 

 from one end of the body to the other. The greater part of 

 it lies between the digestive tract and the imder surface of 

 the body, and is therefore nearest the grotmd when the 

 animal walks. Here again, therefore, all preconceived notions 

 gathered from the familiar vertebrates are upset, as we 

 trace the great nerve-centres, not down the back, but in 



exactly the reverse position. The foremost pair of ganglia, 

 however, which are situated in the head and have been 

 dignified with the name of " brain," as they send nerves to 

 the sense organs, viz. the eyes and antennse, lie above the 

 oesophagus, being connected by two „. 



tluck bands of nervous tissue with ^-'-'^^^~~\ 



the first pair that imderlie it, so en- ~\ Q C 



closing that portion of the digestive V. ,:/* 



tract as in a collar. ^Yhile examin- ]nr 



ing the distended crop as it lies in jUL 



its natural position, a small nervous 2, ^ 



ganglion may be observed as a 

 little triangular whitish spot on its 

 upper surface about half-way down 

 its length ; from this two nerves 

 pass obhquely backwards towards the 

 hinder part of the crop, while a 

 single one rimning forward along the 

 middle line connects this centre 

 with some small ganglia in the neigh- 

 bourhood of the brain. Tliis small 

 collection of nerves and ganglia is 

 called the visceral nervous system. 



Such is the insect which foreign 

 commerce has introduced to our 

 island, and which, by reason of the 

 persistence with which it clmgs to 

 the fortunes of the human race, has 

 become truly cosmopolitan, and seems 

 to be almost a necessary adjunct -r,„ 



, ... . XT 1 -TIG. ii.. i^r,K> v^i o 



ot modern cmhsation. Nowhere a Chain of Cockroach. 

 welcome guest, it yet quietly pushes a. supra - (esophageal 

 on its conquests, and even the deter- S'lngl'i ; '-'■ sub-cesopha- 



mined hostihtv of the tidy housewife f 1'1° =;!,^*l^'"■"'"'°■ ' 



■• ■, . «. abdominal do. 



does not avail to check its progress. 



Its nocturnal habits and love of concealment make it 

 a very difficult insect to eradicate when once it has estab- 

 hshed itself in a house, and it is to be feared that no 

 certain remedy for tliis nuisance has yet been discovered. 

 Spite of " beetle - traps " and " vermin - powders," it 

 maintains its gromid ; neither rats, cats, nor hedge- 

 hogs (all numbered amongst its foes, and the last 

 especially a gi'eedy devourer of it), are able materially to 

 lessen its numbers ; by reason of some subtle superiority, 

 perhaps impossible for our gross senses to perceive, it 

 continues to be \-ictorious over all its enemies, and in the 

 face of all opposition and efforts to exterminate it, still 

 flourishes and continually spreads. It is, indeed, gradually 

 dislodging an old and famihar inhabitant of kitchens, the 

 house-cricket, an insect of very similar habits to itself, and 

 no very (Ustant relation of its own. 



V. orientals is not the only species of cockroach which 

 attaches itself to man. A considerably larger species, 

 F. Americanii, wliich is winged in both sexes, has spread a 

 good deal from its native haimts in Tropical America, 

 and has effected a lodgment in some places in this country. 

 But for some reason or other, it does not seem likely to 

 displace oriniUilis, a curious fact, inasmuch as it is a 

 stronger insect, and, being gifted with wings in both sexes, 

 might be supposed to have had better opportimities of 

 establishing itself. It is a common species on board sliips. 

 An Australian species also appears to be beginning to 

 spread. Again, IHatta iirniuniica, a closely allied form 

 called in America the " Croton Bug," is known all over 

 the United States, and sometimes gets a footing in Britain. 

 In a baker's shop at Leeds it established itself, and is said 

 to have Ijeen introduced by soldiers after the Crimean ^^'ar, 

 coming with them to the barracks, and being thence con- 

 veyed to the bakery in bread baskets. According to 



