Spiders of Missouri

Missouri has over 300 species of spiders, which vary greatly in appearance, habitats, and behavior. All spiders have eight legs, two segments in their bodies, and are covered in short hairs. There are many kinds of spiders, from those that sit in their webs to those that actively chase their prey. 

Spider Anatomy

All spiders have eight legs and two body segments in addition to chelicerae (hollow mouthparts that store venom glands and connect to the spider's fangs) and pedipalps (sensory organs that function much like antennae). Most spiders have eight eyes, although some species have no eyes at all. Eye arrangement is commonly used to identify spiders, although that frequently requires a dead specimen to be able to get the appropriate level of detail.

File:Spider external anatomy main en.png

Spiders and People

Many people are scared of spiders, but there isn't really a need to be afraid of them. Spiders are generally very shy and will run when threatened. Biting is a last resort, and even then, they usually don't release venom when they bite. Venom costs a high amount of energy to produce, so spiders won't release it unless they are trapped or likely to die. Most spider bites with venom are painful but similar in danger to a bee sting-- uncomfortable but not life-threatening.

That said, spider bites can be serious. There are two medically significant spider species native to Missouri, the black widow and the brown recluse. If bitten by one of these spiders seek medical help immediately. It is not worth waiting to see if you have any reaction to the bite. If you are allergic to spider venom, you can have a severe reaction to even non-medically significant spider bites. In general, when bitten by a spider, pay attention to the bite and get help if you are experiencing any form of allergic reaction.

Common Spider Species

Orbweaver Spiders

Orbweavers are a family of spiders that build flat, wheel-shaped webs. In general, they have long legs, large abdomens (particularly in females) and often have noticeable bristles and hairs covering their legs and abdomens.

Verrucosa arenata (triangulate orbweaver)

This small (5-9 mm) orbweaver is commonly found in forests across the eastern U.S. Their webs are circular and are often encountered stretched across trails 4-5 feet above the ground. The exact coloring varies but their abdomens are dark with a light triangle pointing away from the head. This species often sits in the center of the web with their head facing upwards, while most other orb weaver spiders rest with their heads facing downwards. 

Micrathena spiders

Micrathena gracilis (spined micrathena, spiny-bellied orbweaver)

Ground 3 - Micrathena gracilis

Spider in Covington Georgia - Micrathena gracilis

 

There are three different species of micrathena spider found in Missouri. Of these, Micrathena gracilis is the most common, especially in the central and southern parts of the state. These spiders make orb webs between trees and bushes. It is distinguishable from its sister species Micrathena mitrata, the white micrathena, by the number and size of the spines on its abdomen. M. gracilis has 10 very pronounced spines on an abdomen that is elongated and roughly square. They range in color from pale yellow to white with mottlings of brown or black and grow to be 8-10 mm in size. They often hang in their webs with the top of their bodies facing downward and their starkly triangular profile displayed. Males are much smaller (4-5 mm) and thinner and only very rarely seen.

Micrathena mitrata (white micrathena)

Micrathena mitrata - female

Micrathena mitrata is closely related to M. gracilis and behaves in the same manner, spinning orb webs between branches of trees and bushes. Its abdomen is more rounded and has only two spines on its abdomen, both at the end. The roundness of the abdomen varies by individual. As with M. gracilis, their coloration varies from pale yellow to white with brown or black markings on their abdomen. Their size is comparable to that of M. gracilis and the males of M. mitrata are similarly smaller and more rarely encountered, but the males are not thinner in M. mitrata. Instead, their abdomens are often quite rounded as well, although they lack abdominal spines.

Cobweb Spiders

Cobweb spiders (family Theridiidae) are a group of spiders that build three-dimensional, semi-random webs. They have long, thin legs and often quite large and rounded abdomens. 

Parasteatoda tepidariorum (Common house spider)

Spider and babies - Parasteatoda tepidariorum

The common house spider is, unsurprisingly, a common spider found in houses. It lives in all 50 states and usually lives in nooks and corners in people's houses. They have variable markings that consist of brown, tan, and cream spots and stripes in no consistent pattern, as well as dark leg joints. Their legs are long and slender and usually pulled up close to the spider's body, as in the photo above. They are best identified by their egg sacs, which are brown and often crinkled. Males have a much less pronounced abdomen, tend to be redder in color, and are more rarely seen.

Latrodectus mactans (southern black widow)

Latrodectus mactans - female

There are three black widow spiders in North America; of these only the southern black widow lives in Missouri. It is easily recognized by its glossy black body, very large abdomen, slender legs, and bright red hourglass on the underside of its abdomen. The spiders hang upside down in their webs so as to display the hourglass as a warning to potential predators. Males and juveniles lack the hourglass and are also not medically significant. Adult female black widows, however, are medically significant, which means that if you are bitten by an adult female black widow, you should get immediate medical care. The majority of black widow bites are dry, which means that the spider bites but does not inject venom. However, since untreated wet bites (bites with venom) can lead to severe necrosis, it is still a good idea to receive medical attention for a black widow bite.

Cellar Spiders

Pholcus manueli

Cellar Spider - Pholcus manueli - male

This is the only species of cellar spider commonly found in Missouri. They usually live in cool, dark places such as basements or garages. They have extremely long, slender legs and are usually grey to yellow. They are often called "daddy longlegs". They spin webs to catch their prey, although their webs don't have any sticky threads. Instead, their webs are effective by being so tangled that insects that wander into it cannot navigate their way out before the spider attacks it and wraps it in silk. They do not spin silk around their eggs but instead the females protect their egg sacs by holding the loose bundle in their jaws. When threatened, cellar spiders vibrate their bodies rapidly so that predators cannot see where the spider is to attack it.

Wolf Spiders

Dotted Wolf Spider - Rabidosa punctulata - male

Wolf spiders (Lycosidae) are a family of spiders, not any given species. They are active hunters, spinning webs only to rest in and chasing or ambushing their prey. Their coloration varies greatly between species, although they tend to be shades of brown, tan, black, and yellow. The individual species can be virtually impossible to tell apart from each other due to the similarity of coloration and build between species. In order to have a certain identification, you need to examine a dead spider with a microscope. However, you can identify a wolf spider as a wolf spider by looking at the way their eyes are arranged. The row of eyes lowest down on their head, closest to their chelicerae, consists of four small eyes. They have two large, forward-facing eyes above this lower row, and two large eyes mounted on the side of their cephalothorax. Due to this eye arrangement, their cephalothorax is often raised in the front and tapers to a flatter surface near the abdomen. An interesting attribute of wolf spiders is that the females carry their egg sacs with them until they hatch and when they do hatch, she will carry the spiderlings on her back.

Recluse Spiders

Loxosceles reclusa (brown recluse)

Brown Recluse.jpg

Of the 11 Loxosceles spiders in the United States, L. reclusa is the only one found in Missouri. Both males and females are equally common and hard to distinguish, although males have enlarged pedipalps. These spiders, like black widows, are medically significant and can cause severe necrosis, although this is very rare. Because brown recluses hunt actively, that is, instead of waiting for prey to wander into their web they stalk their prey, they are more frequently encountered by humans. However, they are nocturnal, which decreases the odds of coming across them, and they are shy and try to evade human contact whenever possible. Unlike black widows, brown recluses are relatively hard to identify. A common rule of thumb is to look for a fiddle-shaped marking on the cephalothorax pointing to the abdomen, but many other spider species share this characteristic. In order to correctly identify a brown recluse, it must have all of the following traits:

  • Not on a web (brown recluses form webs but are only in them to rest and therefore are out of sight when in their webs, besides which their webs are often hidden)
  • Legs and abdomen with no color variation, stripes, spots, etc.
  • Entire body covered in very short, very fine hair with no spines or bristles
  • Legs that are neither stocky (like a wolf spider) nor spindly (like a cellar spider)
  • Six eyes arranged in three pairs 
  • A fiddle-shaped marking on the cephalothorax pointing towards the spider's abdomen

Research on Spiders

There has been a lot of research on spiders, largely pertaining to their taxonomy distribution, silk production, and ecological niche. One study that I found particularly interesting investigated defensive behavior and intentional control over defensive behavior in Latrodectus hesperus, the western black widow. They prodded and pinched female western black widows and discovered that not only were they less likely to bite when in low-danger situations, indicating an ability to choose whether or not to bite, but in more than half of all the bites delivered, no venom was released. In the instances where venom was released, the amount of venom varied depending on the severity of the threat presented to the spider, indicating an ability to meter out the amount of venom released. (Nelsen, Kelln and Hayes 2014; https://www.sciencedirect.com/science/article/pii/S0003347213005733)

Image sources:

Spider anatomy: https://commons.wikimedia.org/wiki/File:Spider_external_anatomy_main_en.png

Triangulate orbweaver: By Judy Gallagher - https://www.flickr.com/photos/52450054@N04/29703466560/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=54383645

Spined micrathena: https://bugguide.net/node/view/1154077/bgimage

Spined micrathena side view: https://bugguide.net/node/view/1161254/bgimage

White micrathena: https://bugguide.net/node/view/1141481/bgimage

Common house spider: https://bugguide.net/node/view/1215925/bgimage

Southern black widow: https://bugguide.net/node/view/1183313/bgimage

Cellar spider: https://bugguide.net/node/view/1180480/bgimage

Wolf spider: https://bugguide.net/node/view/591035/bgimage

Brown recluse: https://en.wikipedia.org/wiki/Brown_recluse_spider