I have these two bites, one of which just appeared overnight and the other...

Asked December 10, 2017, 8:49 AM EST

I have these two bites, one of which just appeared overnight and the other two nights ago, they’re both blisters and one is big. I was just wondering what I was bit by.

Kalamazoo County Michigan

1 Response


It's difficult to determine what the biter is by looking at the bite. My first guess would be a yellow sac spider , Cheiracanthium inclusum or C. mildei, (Miturgidae).

The following is taken from the Pennsylvania State University, Department of Entomology web site entitled, Commonly Encountered Pennsylvania Spiders (and two rarely encountered but medically important species) at http://www.ento.psu.edu/extension/factsheets/Spider/spiders.htm#yellowsac

Yellow sac spiders can be found walking about on foliage; under leaf litter, stones, and boards; and on buildings under the window sills and siding, in addition to the corners of walls and ceilings within homes. They probably account for more spider bites than any other spider and their bites are sometimes misdiagnosed as brown recluse bites by health care providers. C. inclusum is indigenous to much of the United States (except the northernmost states), while C. mildei, an introduced species from Europe, was found throughout much of the Northeast as of 1978. It is likely that C. mildei has substantially increased its range since that time.

Description: Both species are of similar size (females 5-10 mm; males 4-8 mm) and coloration. C. inclusum is a light yellow to cream color, with the jaws (chelicerae), tips of the tarsi, and palps dark brown. C. mildei has a slightly greenish tinge to its abdomen and a pale yellow cephalothorax. The chelicerae, tarsi, and palps are similar to those of C. inclusum. Both spiders have a slightly darker dorsal stripe running lengthwise across the abdomen.

Yellow sac spider retreats may be found outdoors under objects or indoors in the corners of walls and ceilings. These retreats are silken tubes or sacs in which the spiders hide during the daytime. In homes with light, neutral-colored walls and ceilings, the retreats may go unnoticed, as they are small and blend in with the background coloration.

Life History: Yellow sac spiders deposit their eggs in June or July. The eggs are loosely deposited within a silken retreat, and the female remains nearby to guard them. C. inclusum is more often encountered outside; the majority of these spiders deposit their eggs on the undersides of leaves or other foliage. C. mildei is more often encountered within man-made structures and oviposits almost exclusively indoors. The young spiderlings will often remain within the silken retreat for a short period and eventually venture out at night in search of food. The young will frequently return at daybreak to hide within the protection of the retreat.

Yellow sac spiders are "active hunters," searching for prey rather than capturing it within a web. It is during these nighttime forays that the spiders encounter humans and bite when they become trapped between a person's skin and sheets, clothing, shoes, and the like.

Medical Importance: The yellow sac spiders probably account for more human bites that any other type of spider. The bite of C. inclusum is more destructive then the bite of C. mildei. Humans usually incur C. inclusum bites outdoors while gardening in the summer. C. mildei will readily bite, despite their small size, and they have been observed crawling across the human skin surface and biting without provocation. Fortunately, most bites are relatively painless and do not result in any serious medical conditions. For C. inclusum victims and some individuals sensitive to C. mildei, the bites will exhibit the symptoms described below.

The bite is usually very painful at the outset, with developing erythema, edema, and pruritus. The burning sensation associated with the bite will last for up to an hour, with rash and blistering occurring during the next 1-10 hours. Some patients may exhibit systemic reactions with fever, malaise, muscle cramps, and nausea. These symptoms are similar to black widow bite symptoms but are much less severe. A necrotic lesion and ulceration may also occur at the site, but this is less serious than the similar symptoms that accompany a brown recluse bite, and it usually does not result in scarring.

Sac Spider Control Measures

The following was authored by Susan C. Jones, Ph.D.
Assistant Professor of Entomology

The University of Ohio Publication HYG-2060A-04

Control of sac spiders is best achieved by following an integrated pest management (IPM) approach that involves multiple tactics, such as preventive measures, exclusion, sanitation, and chemicals applied to targeted sites.

IPM requires a thorough inspection of the building to locate the pest and its harborages. This spider can be readily located indoors during the daytime because its silken retreat typically is positioned in upper corners and along the juncture of the ceiling and wall. The spider will emerge when its silk retreat is gently poked with a pencil or similar object. It then can be captured and identified.

Preventing spider bites

  • Shake out clothing and shoes before getting dressed.
  • Inspect bedding and towels before use.
  • Wear gloves when handling firewood, lumber, and rocks (be sure to inspect the gloves for spiders before putting them on).
  • Remove bedskirts. Move the bed away from the wall.


  • Install tight-fitting screens on windows and doors; also install weatherstripping and door sweeps.
  • Seal or caulk cracks and crevices where spiders can gain entry to the house. These cracks and crevices first should be treated with a residual insecticide in case they already harbor spiders.
  • Equip vents in soffits, foundations, and roof gables with tight-fitting screens.
  • Install yellow or sodium vapor light bulbs outdoors since these attract fewer insects for spiders to feed upon.
  • Reduce lighting outdoors.

Habitat Modification & Sanitation

  • Remove debris, wood piles, rock piles, etc. near the foundation that provide hiding places for sac spiders.
  • Eliminate household pests (prey) such as flies, ants, and cockroaches that attract spiders.
  • Do not stack wood against the house.
  • Remove heavy vegetation and leaf litter around the foundation.
  • Prune branches of trees and shrubs so that they do not contact exterior walls and the roof.

Non-chemical control

  • Vacuum using a corner attachment to remove spiders and their silken sacs (dispose of the vacuum bag in a container outdoors). Note it is easy to detect new activity if the sacs are consistently removed.
  • Use a rolled up newspaper or fly swatter to kill individual spiders.
  • Place sticky traps or glue boards on floors along walls behind furniture and large appliances to entangle spiders. However, sac spiders often travel upward, so glue boards are less effective against sac spiders than other active hunting spiders.


There are many labeled pesticides for spider control. Some are labeled for homeowner use, while others are labeled only for the licensed, certified pesticide applicator.

Individual exposed spiders can be killed with aerosol insecticides, but any egg sacs will be unaffected. It generally is best to use a vacuum cleaner so that the egg sac within the silken retreat is removed from the premises.

A residual insecticide, such as Bayer Home Defense or Ortho Home Defense Max, can be applied to corners, behind and under furniture, behind stored items, etc. to control active hunting spiders. This approach also is useful to prevent establishment of new spiders. Aerosol flushing agents and insecticide foggers containing pyrethrins, though ineffective by themselves in providing long-term control, can cause spiders to move about so that they contact treated surfaces.

Residual liquid sprays can be applied to the outside perimeter of the home, but control is often achieved secondarily in such cases because the sprays eliminate the sac spiders prey, thereby reducing the number of spiders.