416 ORDERS OF PTILOTA. 



quite inexplicable, except on the supposition that an adhesive 

 secretion is emitted by the instruments employed in climbing. 

 'I'ln in-xt 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 visi- 

 ble 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 conjecture 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 cir- 

 cumstances. Now, it is reasonable to infer, from the fore- 

 going researches, that the hair-like appendages constituting 

 the brushes of spiders, and occurring in such profusion on 

 the interior surface of the pulvilli of insects, are tubular." 



The larvae of the Diptera, as well as the perfect insects j 

 have characters peculiar to themselves. In many of these 

 larvie the head is of a fleshy substance, without any determi- 

 nate form, whilst in the majority of the larva; of other orders 

 the head is horny and consistent in form. The breathing 

 pores have also a peculiar disposition ; instead of being placed 

 in pairs upon the first, fourth, and following segments of the 

 body, as is generally the case, the anterior pair are found 

 upon the second segment, whilst all the rest, from two to 

 eight in number, are brought together upon the terminal seg- 

 ment. Those larvae, which are constantly footless, or but 

 rarely provided with fleshy appendages, have the mouth armed 

 with two points formed 1'or piercing the matters upon which 

 they feed. In the transformations which these insects uu- 



