The black widow spider, belonging to the genus Latrodectus, is highly recognizable due to the female’s appearance and reputation. Distinguishing between the sexes is important because the female is the only one that possesses fangs large enough and venom potent enough to pose a medical threat to humans. Her venom contains alpha-latrotoxin, a neurotoxin. Knowing the distinct physical characteristics of the adult male, adult female, and juveniles is the key to correctly identifying any spider encountered.
Identifying the Adult Female
The adult female black widow is significantly larger than the male, with a body length typically measuring around 12 to 13 millimeters. Her overall appearance is striking, characterized by a smooth, glossy, jet-black exoskeleton across her entire body and legs. This deep black coloration is a primary identifier.
The most famous and defining feature is the bright red or reddish-orange hourglass marking located on the underside of her large, globular abdomen. While often a complete hourglass shape, it may sometimes be broken into two distinct, separated spots or triangles.
The female’s robust size and dark coloration are directly related to her ability to deliver venom. She has larger venom glands and fangs compared to the male, allowing her to inject the neurotoxin. She will only bite defensively if she feels threatened or disturbed, especially if guarding an egg sac.
Identifying the Adult Male
The adult male black widow is dramatically different in appearance from the female. Males are extremely small, measuring only about 3 to 6 millimeters in body length. Their smaller size means their fangs are too small to effectively pierce human skin, rendering them medically harmless.
Unlike the female’s solid black color, the male is typically lighter, often exhibiting shades of brown, gray, or a paler black. His abdomen frequently features a pattern of white or reddish stripes and spots, a stark contrast to the female’s glossy, uniform black. The male’s legs also appear disproportionately longer and more slender.
A definitive feature of the mature male is the structure of his pedipalps, the two small, leg-like appendages near his mouth. These pedipalps become swollen or bulbous at the tips, resembling tiny boxing gloves. These structures are used for transferring sperm during mating, and their enlarged appearance is a clear indicator of a mature male.
Identifying Juvenile Spiders
Juvenile black widow spiders are often the source of confusion because their appearance does not yet match the iconic adult female. When they first hatch, they are very small, around 3 millimeters, and are generally light-colored. As they grow, they undergo multiple molts, gradually acquiring more dark pigmentation.
The markings on juveniles are quite distinct from the adult female’s simple hourglass. Their abdomens are often covered in a mottled pattern of various colors, including white, yellow, orange, and brown, along with multiple spots or stripes. These markings gradually consolidate and change with each molt, eventually forming the classic red hourglass on the female as she reaches full maturity.
Distinguishing gender in these immature stages is virtually impossible for the average observer, as both male and female spiderlings share this varied coloration. The only reliable way to sex a black widow is by observing the size, color, and specific morphological structures of the fully mature adult form.