1) spiders: "Most spiders that hunt actively, rather than relying on webs, have dense tufts of fine hairs between the paired claws at the tips of their legs. These tufts, known as scopulae, consist of bristles whose ends are split into as many as 1,000 branches, and enable spiders with scopulae to walk up vertical glass and upside down on ceilings. It appears that scopulae get their grip from contact with extremely thin layers of water on surfaces. Spiders, like most other arachnids, keep at least four legs on the surface while walking or running." Source and further information: http://en.wikipedia.org/wiki/Spider#Locomotion "Coated with a waxy layer, the spider's exoskeleton is water repellant and has extremely low adhesion, which means that spiders don't just stick to whatever they happen to be on. Though this might seem a limitation for a spider wanting to crawl across the ceiling, in fact a much more complex and sophisticated system has evolved to enable it to do that. A spider has claws that it can use to cling to a roughly textured surface, but on a smooth surface a wonderfully adapted system of microscopic proportions on the spider's feet enables it to hold on. The precise nature of this "anti-gravity" attachment system was the subject of the work done at the University of Zurich, the first study of its kind. The spiders studied at Zurich are members of a family of jumping spiders that hunt down prey without building a web, so holding onto surfaces while carrying significant weight (i.e., food) is obviously crucial to their survival. On each of the spider's feet there are hair-like tufts, called scopulae, and using a scanning electron microscope it was discovered that a single scopula is itself composed of many, many, much smaller, single hairs. It is these minuscule hairs, or setules, that actually represent the direct contact points with a surface. The number of setules per foot is estimated to be 78,000 each, and since spiders have eight feet, they have upwards of 600,000 individual points of contact with any given surface. The ability of the spider to cling to overhead smooth surfaces is due to lots and lots of extremely miniaturized contact elements, or as the scientists explain: "Branched hairs and progressive structural miniaturization, broadened contact elements, as well as the absence of adhesive secretions, are characteristic features of ... the spider attachment system." Underlying this system of course, are principles of physics. Though each of these points of contact is extremely small, together they combine into a powerful force. In this case they are known as van der Waals forces. Essentially, the spider's leg hairs and the surface it is sitting on are bonding with each other, at the molecular level. The ceiling is holding on to the spider and the spider is holding on to the ceiling. Extremely week individually, when all of these forces act in concert on these hundreds of thousands of hairs, the total adhesive force is extremely powerful, up to 170 times the weight of the spider, if all eight legs are in contact. Jumping spiders however, as their name would indicate, rarely have all eight legs on a surface at once, yet even one leg can exert approximately enough force to support 21 times the weight of the spider. It's not hard to start thinking about how much a person could carry if they wore a suit that exerted roughly the same degree of force. A lot of pizzas! Due to the power of the van der Waals forces, "adhesion becomes independent not only of material characteristics but also of conditions of the surroundings," surroundings such as a vacuum or a weightless environment like outer space, or even underwater." Source and further information: http://www.istl.org/05-summer/article3.html Further information: http://www.answerbag.com/q_view/74454 2) flies: "The flies can walk on vertical planes, and can even hang upside-down from ceilings. This is accomplished with the surface tension of liquids secreted by glands near their feet." Source and further information: http://en.wikipedia.org/wiki/Housefly Further information: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T3F-48S2YH3-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=36e2393f6bf3d16d42ae1250993a9d49 3) geckos: "The ability of geckos to climb on sheer surfaces has been attributed to van der Waals force.[3] A recent study suggests that water molecules of roughly monolayer thickness (present on virtually all natural surfaces) also play a role. Nevertheless, a gecko can hang on a glass surface using only one toe." Source and further information: http://en.wikipedia.org/wiki/Van_der_Waals_force 4) Ants: "A hooked claw at the end of each leg helps ants to climb and hang onto surfaces." Source and further information: http://en.wikipedia.org/wiki/Ant 5) cockroach: "The segmented tarsus acts like an ankle and foot. The hook-like tarsus also helps roaches climb walls and walk upside down on ceilings." " When it runs upside down on a ceiling, it takes longer steps in an attempt not to fall down. In fact, it takes significantly more energy for a roach to run upside down than to run up a vertical wall." Source and further information: http://animals.howstuffworks.com/insects/cockroach1.htm

Much is made of special mechanisms of adhesion between their feet and the surface -- this indeed keeps them falling by gravity. But a major factor often ignored is their small size. Due to the physics of scaling factors, a bug has such a tiny weight that gravity simply doesn't dominate their world as a force to be overcome compared to humans or other large animals. If humans were as small as ants, we wouldn't notice gravity much, either. This is due to the fact that weight rises as the third power of length. A creature only 1/100 (one percent) the SIZE of another has 1/1,000,000 (one millionth) the WEIGHT.