The female's body is 14–16 mm (1/2 in) in length and is black, often with an hourglass-shaped red mark on the lower abdomen.

The male of the species is around half this length and generally a tan color with lighter striping on the abdomen.

Female black widows have potent venom containing a neurotoxin active against a range of mammals (see latrodectism).

[3] The female's consumption of the male after courtship, a cannibalistic and suicidal behavior observed in Latrodectus hasseltii (Australia's redback),[4] is rare in this species.

[10] The time spent on courting varies tremendously and can be as short as ten minutes or as long as two hours.

Male's genital bulbs may get injured or partially destroyed in the act of copulation, which leaves him unable to mate again in the future.

The female receives chemical cues given off by other nearby L. hesperus that indicate the population density, and therefore give them a notion as to whether their choosiness will pay off or fail to do so.

As a by-product of this detection, L. hesperus is more likely to build a home in an area that is expressing a high degree of chemical prey cues.

In one study in British Columbia, the orders Coleoptera and Hymenoptera accounted for the majority of the spiders' prey; the majority of prey within Coleoptera were darkling beetles, weevils, and ground beetles, and the most common Hymenoptera were ants, wasps, and bumblebees.

[15]The peak activity of hymenopterans in this study was May through September in British Columbia, paralleling a spike in consumption by L. hesperus during these months.

There are three circumstances under which L. hesperus may feed on conspecifics: when alternative prey options are scarce; when a spider is very hungry; in self-defense during antagonistic bouts.

Specifically, within siblingships the range of time lapsed before the spider siblings fell victim to cannibalism varied from two days to three weeks.

There appears to be a direct relationship between the length of time a L. hesperus will last before falling back on sibling cannibalism and the amount of maternal investment put into their egg sac.

In other words, the more investment a mother puts into her egg sac, the longer their offspring will go before engaging in sibling cannibalism as their source of prey and nutrition.

[18] The droplets are about twenty times larger than the center fiber's diameter and are visible to the naked eye.

[20] Latrodectus hesperus' silk contains sexual pheromones which are perceived by the opposite sex upon contact with the web.

When the spider reaches satiety, it will decrease its energy expenditure on building the features of the web specifically purposed for prey capture.

Although there is a decrease in prey-capturing aspects of the web, the overall silk production remains steady or in some cases will increase.

[18] Juveniles and female adult L. hesperus can eject a chunk of viscid silk toward potential predators.

[18] However, adult males are unable to produce this defensive silk, presumably because they need to conserve energy for reproduction during their relatively short life span.

[18] Since the spider will stay largely stationary at the web hub, such attacks are mostly successful if there is no defensive behavior.