EXPERIMENTS ON FLIES. 



397 



repeat Mr. Black wall's observations, and extend the inquiry. He 

 was led subsequently to make the following experiments. 



(i Clean phials of transparent 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. 

 Thes,e facts, far from corroborating the mechanical theory, appeared quite inex- 

 plicable, 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 of moderately high magnifying powers, 

 in a strong light, and at a favourable angle, speedily convinced me that my con- 

 jecture was well founded, as I never failed to discover unequivocal evidence of its 

 truth ; though, in the case of the spiders, considerable difficulties presented them- 

 selves, in consequence of the exceedingly minute quantity of adhesive matter 

 emitted by the brushes of those animals. On submitting this secretion to the 

 direct rays of the sun, in the month of July, and to brisk currents of air, whose 

 drying power was great, I ascertained that it did not suffer any perceptible 

 diminution by evaporation under those circumstances. 



" Now it is reasonable to infer, from the foregoing remarks, that the hair-like 

 appendages constituting the brushes of spiders, and occurring in such profusion on 

 the inferior surface of the pulvilli of insects, are tubular. The delicate membrane 

 also, on the under side of the pro-legs, and the tarsi of the perfect legs of various 

 larvae capable of traversing polished perpendicular bodies without the aid of lines 

 produced by a spinning apparatus, must be provided with numerous pores, or minute 

 papilla, from which an adhesive secretion is emitted. Some larvce which are not 

 supplied with pro-legs, those of coccinellce for example, have the inferior part of the 

 tarsi of their perfect legs thickly covered with hair-like appendages, resembling in 

 figure and in the function they perform, those on the pulvilli of insects in the imago 

 state ; while others, altogether destitute of legs, emit a viscid mucus from both 

 their extremities, and by advancing and attaching each alternately, are thus enabled 

 to ascend smooth bodies with facility. 



" According to my observations, the instrument is composed of several branched 

 membranous papilla included in a common envelope. They are extremely flexible 

 and extensile, and, either separately or collectively, can be protruded beyond the 

 caudal segment, or retracted within it at the pleasure of the animal. Their efficacy 

 as a cleaning apparatus and an organ of adhesion and progression depends princi- 

 pally upon the mucus they emit, which is secreted in great abundance, and not 

 upon the power of producing a vacuum. When this instrument is applied to the- 



