Differences Between Spiders and Insects
Feature Spiders Insects
Main body parts 2 3
Walking legs 8 6
Eyes Simple Compound
Jaws Piercing (fangs) Chewing
Antennae No Yes
Ability to fly No Most
Abdominal silk organs Yes No
Figure 1. This Eocene-age fossil spider is a male, based on the swelling of the pedipalps. Many fossil spiders are impressions that are barely discernable. Florissant Fossil Beds National Monument specimen number 2971A. Photo: R. Woods.
It is difficult to assign Florissant’s fossil spiders to a genus and species based on their external features observed in the paper shales. Since microscopic characteristics cannot be seen in the fossil impressions at Florissant, an outline morphometric study, using carapace (dorsal exoskeleton) shape and leg characters, has been effective in making family placements of fossil spiders (Kinchloe, 2004).
All spiders have two well-divided body sections: the cephalothorax (prosoma) followed by an abdomen (opisthosoma) (Grimaldi, 2005). The abdomen contains the digestive and reproductive systems and on the ventral surface near the apex are spinnerets that deliver small threads of silk. Silk has many functions such as making intricate webs that capture prey, encasing eggs, and building elaborate nests or burrows. Spiders with a distinctive silk organ, called a cribellum, are included in a special group called the Cribellatae. The fossil spider in figure 1 has a cribellum and belongs to this group (Rasnitsyn, personal communication, 2006). The specimen in figure 1 has not yet been classified beyond the order.
Taxonomic Classification of Spider in Figure 1 (It is difficult to classify some of Florissant’s fossil spiders into higher taxonomic categories)
Order Araneae (spiders)
Spiders have eight walking legs, all attached to the cephalothorax. On the front of the cephalothorax are the mouth, fangs to bite prey with, and eyes. The first pair of appendages—the chelicerae—are used for piercing, handling prey, and injecting venom. The second pair of appendages, the pedipalps, are used for mating and are much larger in male spiders than in females.
All species of spiders are predatory—spiders that do not spin webs, such as wolf spiders and tarantulas, stalk or ambush their prey. Spiders feed by a process known as external digestion. When spiders catch an insect, they inject venom that paralyzes their prey. The spider’s venom also contains digestive enzymes. These enzymes liquefy most of its victim’s insides so the spider can feed on this mixture of nutrients (figure 2).
Figure 2. Yellow Garden Spider (Argiope aurantia), Wellington, Kansas. Photo: © 2003 Joseph Hall. Used with permission by Joseph Hall.
Grimaldi, D and Engler, M. S., 2005, The Evolution of Insects: New York, Cambridge University Press, 689 p.
Kinchloe, A. E., 2003, A taxonomic study of the Eocene spiders from Florissant, Colorado: Geological Society of America Abstracts with Programs, v 35, no. 6, p. 537.
Kinchloe, A. E., Smith, D.M., Cushing, P. E., and Gurlalnick, R., 2004, A morphometeric study of the Eocene spiders of Florissant, Colorado: Geological Society of America Abstracts with Programs, v 36, no. 5, p. 40.
Meyer, H.W., 2003, The Fossils of Florissant: Washington, D.C., Smithsonian Books, 258 p.
Rasnitsyn, A. P., [Russian Academy of Science], personal communication, June, 12, 2006,