Remember, spider — not an insect. Spiders differ including a special body structure, and your task — to clear it. The body of a spider consists of a cephalothorax, abdomen and legs. Between the cephalothorax and abdomen of the spider has a kind of "waist", about how wasps — the figure of the cephalothorax and the abdomen should be separated. Form and abdomen, and the cephalothorax is rounded, different species of spiders it is different, but these differences are not significant for you, if you are a professional artist and do not seek to illustrate a biology textbook.
The number of legs spiders are clearly distinguished from the other species. The legs of your spider should be eight. In principle, the cephalothorax of a spider , there is also the so-called pedipalpy, nogoshchupaltsa — these bodies spider touches all around, but for a clear recognition of the spider enough image eight feet.
If you aspire to a schematic drawing, we can restrict the already mentioned details of spider silhouette. For greater realism, you can add the eyes — some species of spiders eight of them, some less, and some absent, and jaw. Jaw spider powerful, because spiders are predators, and so on your drawing, they must be clearly marked, if you want to detail.