The most accurate way to determine a spider’s species is to look at it under a microscope and examine the male and female reproductive organs. In the field, this is obviously not practical, and a field guide must rely on more obvious features that can help with identification down to at least the family, and sometimes the genus, level. There are a few spider species, however, which are distinct enough from any others that an experienced guide can identify the exact species at a glance.
In the field, there are several factors one must consider when attempting to identify a spider: habitat, web-bound or free-living, web-type, found on the ground or vegetation or near water, general impression of shape, colour, and size, and most importantly, the eye pattern.
All spiders have 2-8 simple eyes and the pattern of their eye arrangements is usually unique from one genus to another, even within the same family.
For example, a wolf spider (Family Lycosidae) will have two large eyes on the top and to the sides of the head, another two large eyes are at the forward facing, and another four smaller eyes are forward facing in a horizontal line just below. A baboon spider (Family Theraphosidae) however has eight small eyes all in a tight cluster on the top of its flat carapace on a little bump, or protuberance.
So, although some wolf spiders can grow almost as large as a baboon spider, and can be hairy like a baboon spider, the eye pattern is distinct enough between the two that the two families should never be confused.
See Separate posts below for Baboon Spiders and Wolf Spiders for photo illustrations of eye patterns.
The identity of a web-living spider can also be narrowed down just by looking at the type of web it lives in. There are 4 main types of webs:
- - Orb Webs: widely considered to be the “standard spider web”. They are generally round-ish, with radials spreading out from a hub in the centre, all hanging from a “bridge line” between two anchor points (often on trees or branches). Example: Golden Orb Web Spiders (Family Nephilidae)
- - Reduced or Modified Orb Webs: variations on the orb web theme, these webs sometimes do not resemble an orb at all. Example: Tropical Tent-Web Spiders (Family Araneidae)
- - Sheet Webs: Horizontal webs constructed in a sheet. The silk is usually not sticky like that of an orb web. Example: Grass Funnel-Web Spiders (Family Agelenidae)
- - Space Webs: Complex, 3-dimensional webs that appear in a variety of different configurations and habitats. Example: Button/Widow Spiders (Family Theridiidae)