158 



KNOWLEDGE ♦ 



[June 1, 1889. 



opened blades of a pair of scissors. In speaking of an 

 insect's mouth organs, just as of those of a spider, a crab, or 

 a lobster, or any animal allied to these, it is necessary con- 

 stantly to bear in mind that the jaws are totally different 

 from the parts similarly named in a vertebrate. They are, 

 in fact, extei'nal appendages, constituted on the same type as 

 the limbs, though usually very greatly modified in form, and 

 difiering iu the degree of development of the several parts ; 

 their movement also is lateral or transverse — i.e , from the 

 sides towards the middle, never longitudinal, as in a 

 vertebrate. If this be borne in mind, it will be obvious 

 that weapons such as the mandibles of a tiger-beetle are 

 eminentl}' adapted for the capture of living prey, but that 

 without some other organs to come to their assistance they 

 would be of sma'il use either for- tearing it to pieces or for 

 conveying the fragments to the mouth. No object secured 

 between their tips could be brought near the mouth, for, as 

 they are hinged at the base, and their tips describe the arc 

 of a circle, it would obviously be necessary, in order that 

 this should be practicable, that the length of each mandible 

 should not be greater than half the distance their bases are 

 apart. But this maximum is greatly exceeded in the 

 present instance, so that when closed they cross at about 

 the middle of their length. The difficulty is obviated by 

 means of another pair of so-called jaws, the maxilla>, more 

 complex in structure than the mandibles, but at the same 

 time far weaker ; to these, which are placed immediately 

 beneath the mandibles, and are furnished with a pair of 

 jointed appendages, the maxillary palpi, is assigned, in 

 concert with a central hinged plate bounding the mouth 

 opening above, and a similarly placed piece bounding it 

 below, the duty of manipulating the food, assisting to pinch 

 off portions of it after it has been alraidy mangled by the 

 mandibles, and guiding them into the aperture of the 

 mouth. The plate which guards 

 the mouth above is called the 

 labrum (fig. 3), and Ln these 

 insects is enormously deve- 

 loped, forming a kind of lid 

 lapping over the base of the 

 mandibles to a considerable 

 extent, and as it is of a 

 yellowish white colour, and 

 thus contrasts strongly with 

 the prevailing green of the 

 rest of the body, it forms one 

 of the most striking and con- 

 spicuous features of the insect. The piece which bounds 

 the aperture of the mouth below is called the labium, and. 

 like the maxillse, is furnished with a pair of long palpi 

 fringed with hairs. The combined action of all these 

 organs, which, so to speak, play into one another's hands 

 like the parts of a complicated machine, render it almost 

 impossible that any poi'tions of food should fail to reach 

 their destination. 



In picturing to ourselves, then, the taking of a meal by 

 this tiger of the insect world, we are not to think of a series 

 of operations like those of its name.sake in the vertebrate 

 sub- kingdom, we are not to think of the prey as held down 

 by the fore feet of its captor, while a pair of internal and 

 vertically moving jaws, carrying teeth imbedded in sockets, 

 are tearing it to pieces, the head meanwhile being moved 

 about with the utmost freedom from one side to the 

 other, as the position of the morsel may demand ; but 

 we are to imagine the prey secured between two sickle- 

 shaped, external and laterally moving jaws, hinged to a 

 he;id which, in consequence of the hard and inflexible arma- 

 ture with which it is covered, has but little freedom of 

 movement, and carrying on their inner edges teeth which 



Fig. 3. — Labrum of Tigeb- 



Beetle. a. Line of junction 



with head. 



are not imbedded in sockets, but are simple projections of 

 the substance of the jaws themselves, and while thus held 

 and pinched, we are to imagine it further as played upon by 

 another pair of laterally moving external appendages beneath 

 the jaws, assisted by a central movable plate both above 

 and below, as well as by two pairs of jointed, finger-like 

 palpi, the food being, by the combined exertions of all these 

 organ.s, gradually reduced to sufficiently small fragments, 

 and passed into the aperture which constitutes the real 

 mouth. 



Cicindda campestris is a plucky insect, and if captured, 

 will make sturdy efforts to iuflict a wound on its captor by 

 aid of its mandibles ; but though these are terrible enough 

 weapons to any unlucky insect that may fall in their way, 

 the) are not powerful enough to do harm to human kind, 

 and can only inflict a sharp pinch. In one sense a tiger- 

 beetle is a pleasant thing to hold in the hand, for it emits a 

 powerful and very agreeable scent, which has been variously 

 described as like that of roses, sweetbriar, or crushed 

 verbena leaves ; no such description, however, can give an 

 entirely adequate notion of it, and as with most other 

 scents, the only way to understand what it really is like, is 

 actually to smell it. It is rather curious that a carnivorous 

 animal should be pleasantly fragrant ; one usually associates 

 odours the reverse of delightful with flesh-eating habits, and 

 most of the other group? of carnivorous beetles fully bear 

 out such a generalisation. A somewhat parallel case, how- 

 ever, is to be found amongst the mammalia, in the civets 

 and genets, which are carnivorous, and are noted for the 

 strong perfume that accompanies them, a perfume so strong 

 indeed as to become almost repulsive, and it may seem not 

 far short of an insult to the tiger-beetle to suggest a com- 

 [larison. It is another curious coincidence that, as with the 

 tigers and leopards and the rest of the Felidce amongst 

 mammals, we have in these insects an exceedingly handsome 

 coloration associated with the highest development of 

 carnivorous propensities. 



{To he continued.) 



THE NATION'S PURSE. 



By Ale.x. B. MacDowall, M.A. 



HE graphic method does not .seem to have 

 been much applied as yet to the financial 

 statements of the nation. But it is capable 

 of aflbrding a useful coupi d'ccil of the con- 

 dition of finances in a .series of years. We 

 propose to illustrate this month the national 

 !^ revenue and expenditure. 

 If the average British citizen and voter were required to 

 state off hand how we got our money and how we spent it, 

 in what proportions this or that source contributed to the 

 whole national income, how much we spent on the debt, 

 how much on the army and navy, and so on, and how our 

 income and outlay have varied in the last twenty or thirty 

 years, his answers would, in many cases, -ve fear, " leave 

 something to be desired " in the matter of precision. 



Our two diagrams, attentively considered and fixed in the 

 mind, might possibly help him. The course of a curve, and 

 its relitive position among other curves, may be remembered, 

 where long reading or recital of figures leaves comparatively 

 no impression. 



These curve pictures show the chief items of revenue 

 and expenditure each j^ear since 1870. The total revenue 

 has risen from 75 to 88 millions ; the total expenditure from 

 68 to 87 millions (roughly speaking). As to the revenue, 

 we may point out that excise is the greatest source. 



