D I P T E R A. 



289 



the close application of the margins of these soles of 

 the feet as we may term them, and the subsequent 

 muscular raising 1 up of their central parts ; but the 

 following remarks by one of our most acute modern 

 observers, Mr. Blackball, published by him in the 

 Appendix of the last volume of the Linniean Trans- 

 actions (in correction of a memoir previously pub- 

 lished by himself, in which he had adopted the 

 vacuum system), will be read with interest, as showing 

 what interesting sources of inquiry are opened to the 

 student of nature, even in the commonest objects of 

 the creation. 



In experimenting upon the house-fly, he observed 

 that individuals frequently remained fixed to the sides 

 of an exhausted glass receiver after they had entirely 

 lost the power of locomotion, and an evident disten- 

 sion of the abdomen had been occasioned by the 

 exhaustion of the aeriform fluids it contained. To 

 detach them from these stations, the employment of 

 a small degree of force was found requisite. 



" In prosecuting this subject, clean phials of trans- 

 parent glass, containing spiders and various insects 

 in the larva and imago states, capable of walking on 

 their upright sides, were breathed into, till the 

 aqueous vapour expelled from the lungs was copiously 

 condensed on their inner surface. The result was 

 remarkable ; the moisture totally prevented those 

 animals from obtaining any effectual hold on the 

 glass, and the event was equally decisive if a small 

 quantity of oil was substituted for the aqueous 

 vapour. A similar consequence ensued, also, when 

 the flour of wheat, or finely pulverised chalk or 

 gypsum, was thinly strewn on the interior surface of 

 the phials, the minute particles of those substances 

 adhering to the tarsal brushes of the spiders, the 

 pulvilli of the perfect insects, and the under side of the 

 feet of the larva?. These facts, far from corroboratin 

 the mechanical theory, appeared quite inexplicable, 

 except on the supposition that an adhesive secretion 

 is emitted by the instruments employed in climbing. 

 The next point to be determined, therefore, was, 

 whether spiders and insects in the larva and imago 

 states, when moving in a vertical direction on clean 

 glass, leave any visible track behind them ? Careful 

 and repeated examinations, made with lenses ol 

 moderately high magnifying powers, in a strong 

 light, and at a favourable angle, speedily convincet 

 me that my conjecture was well founded, as I never 

 failed to discover unequivocal evidence of its truth 

 though, in the case of the spiders, considerable diffi- 

 culties presented themselves in consequence of the 

 exceedingly minute quantity of adhesive matter 

 emitted by the brushes of those animals. On sub- 

 mitting this secretion to the direct rays of the sun, h 

 the month of July, and to brisk currents of air, whose 

 drying power was great, I ascertained that it did noi 

 suffer any perceptible diminution by evaporation 

 under those circumstances. Now, it is reasonable to 

 infer, from the foregoing researches, that the hair-like 

 appendages constituting the brushes of spiders, am 

 occurring in such profusion on the inferior surface o 

 the pulvilli of insects, are tubular." 



The larvae of the diptera, as well as the perfec 

 insects, have characters peculiar to themselves 

 In many of these larvae the head is of a flesh) 

 substance, without any determinate form, whils 

 in the majority of the larvae of other orders the 

 head is horny and consistent in form. The breathing 

 pores have also a peculiar disposition ; instead o 

 NAT. HIST. VOL. II. 



eing placed in pairs upon the first, fourth, and 

 ollowing segments of the body, as is generally the 

 :ase, the anterior pair are found upon the second 

 egment, whilst all the rest, from two to eight in 

 lumber, arc brought together upon the terminal 

 egment. Those larvae, which are constantly foot- 

 ess, or but rarely provided with fleshy appendages, 

 lave the mouth armed with two points formed 

 bi piercing the matters upon which they feed. In 

 he transformations which these insects undergo in 

 heir passage to the perfect state, there is less diver- 

 sity than in the organisation of the perfect insect, 

 [nsects in general are oviparous, but there are of 

 course exceptions to this rule, and two of the most 

 remarkable are to be found in this order. We have 

 already, in our article upon the BLOWFLY, detailed 

 one of these exceptions, in which the eggs are 

 hatched in the body of the parent, and produced in a 

 living state. The other occurs in the forest-flies 

 [Hippoboscida:), in which group not only are the 

 iggs hatched within the body ot the female, but the 

 arva there acquires its full growth, and assumes the 

 pupa state, being ejected from it in the shape of an 

 egg as large as the abdomen of the parents, and from 

 which the perfect insect, instead of the larva, makes 

 its escape. 



The larvae of the diptera in general offer so great a 

 simplicity of structure, as to cause an uniformity of 

 appearance ; nevertheless, those which reside in the 

 water arc more diversified in their characters, being 

 furnished with organs of nutrition and respiration 

 very unlike those of the terrestrial larvae ; for an 

 account of which we must refer to our articles upon 

 CHIUONOMUS, CULICIDA:, COKETHRA, &c. 



In passing to the pupa state, these larva? employ 

 two principal modes. In the greater number there is 

 no shedding of the skin ; the skin of the larva 

 hardens, contracts, and becomes an oval cocoon, 

 within which the pupa is disengaged, appearing at 

 first merely as a gelatinous mass, but afterwards 

 exhibiting in some degree the different parts of the 

 perfect insect, the eyes and wings being folded upon 

 the breast. In the others the metamorphosis is 

 effected by the larva shedding its skin, and the insect 

 then appearing in the form of an inactive incomplete 

 pupa, like that of the former, but not inclosed in a 

 cocoon. In some of the latter, as in the aquatic 

 species, the pupa retains its activity, jerking about 

 with much agility in the water, whilst many are quite 

 inactive. 



In their perfect state, the insects of this order are 

 scarcely less numerous, in point of species, than any 

 other order of insects ; but, if we look at them with 

 respect to the number of individuals, we find them 

 infinitely to exceed any other. The clouds of midges, 

 for instance, rising at eventide over the marshes, like 

 the incense of the Temple, equally pay homage to 

 the Divinity, in showing forth His mighty power; 

 whilst myriads of flies of every kind are to be found 

 in every quarter of the globe, traversing plants and 

 every animate object, and more particularly all that 

 has ceased to live. 



This immense profusion with which they are 

 dispersed over the globe causes them to fulfil two 

 very important functions in the economy of nature ; 

 first, they serve for food to a vast number of the 

 higher animals. Charged with a Divine mission by 

 Him " who giveth food to the young bird," the swallow 

 and the sparrow destroy them by myriads, and the 

 T 



