Spiders are not insects
Contrary to insects, spiders use a dry adhesive system for attachment to smooth surfaces such as glass of plant leafs. The absence of sticky fluids requires the spiders to employ a different strategy which consists in a fine structural differentiation of the skeleton material into hairs (setae) from which even tinier structures emerge (setules). These setules are brought into close contact with the surface and hence adherer via van der Waals forces. But no matter how strong the attachment is, the spiders easily detach their feet from any surface. This is achieved by an asymmetric design of the setule-tips and different leg kinematics during touch-down and lift-off. While web-living spiders catch their prey by waiting, freely hunting spiders have to literally hunt their prey. Jumping off a smooth surface, catching the prey and subsequently holding on the smooth surface again would not be possible without such an elaborated system of attachment as spiders have developed.