The Common Spiders of the United States

by J. H. Emerton

THE GENUS THERIDIUM

The Theridiums are small soft-bodied spiders, making large and loose webs without any large flat sheet of silk, but only a slightly closer portion where the spider stands, or a nest or tent connected with the web. Theridium tepidariorum [(fig. 258)] and rupicola [(fig. 261)] live in houses or among rocks, making large loose webs, in which the spider often stands without any covering. They have the abdomen high in front and tapering a little toward the spinnerets. Theridium globosum [(fig. 262)] has the abdomen of the same shape. The other species are all small and have the abdomen round and brightly colored. They live in more open places on plants, where they make nests in which they are partly hidden, and carry their webs over the neighboring leaves and twigs (figs. [253], [254]).

Theridium tepidariorum.—This is one of the most common house spiders, and is often found in its webs among rocks, but seldom on plants. The females [(fig. 258)] measure sometimes over a quarter of an inch in length, but may mature much smaller. The legs of the first pair are nearly three times the length of the body. The male [(fig. 259)] is shorter and has longer legs. The color varies from dirty white to almost black. The cephalothorax is yellow brown, and the legs light yellow, with brown or gray rings at the ends and the middle of the joints. In the males the legs are orange brown, darker at the ends of the joints. The abdomen is high in front and narrows toward the spinnerets. In dark and well-marked specimens the abdomen has, on the hinder part, six transverse black marks curved upward, thicker in the middle, and partly connected by black spots at the ends [(fig. 260)]. These marks are most sharply defined on the hinder edge, where they are bordered by silver white. The upper mark often forms a conspicuous black and white spot in the center of the abdomen. In light individuals all the markings are smaller and less definite.

It makes a large web in the corners of rooms, under furniture, and in the angles of fences and between stones [(fig. 255)]. It usually stands in the most sheltered part of the web, where a part of it is more closely woven than the rest, but not enough so to conceal the spider. It occasionally makes the web in an open place where there is no shelter above, and then it sometimes carries a piece of leaf into the web and hides under it, as is the usual habit with some allied species. The webs of the young are usually more regular in form than those of adults (figs. [256], [257]). A male and female often occupy the same web for a long time. The eggs are laid in brownish pear-shaped cocoons, several of which are made in the same season by one spider and hang in the web. This species is found all over the world.

Theridium rupicola.—This resembles closely tepidariorum and is easily mistaken for the young of that species. It does not grow larger than an eighth of an inch long. The colors are like tepidariorum, usually dark gray with black spots, the back of the abdomen sometimes almost white. The legs are distinctly ringed with light and dark. In the middle of the abdomen is a pointed hump, the front part generally black and the hinder part white [(fig. 261)].

It lives under stones and among rocks, in webs like those of tepidariorum, often containing grains of sand which look as if placed there by the spider, as sand falling into such a web would go through without sticking to the threads.

Theridium globosum.—This is another species with a high abdomen like tepidariorum. It is about a twelfth of an inch long and almost as high [(fig. 262)]. The abdomen is a little flattened behind and pointed toward the spinnerets. The hinder part is white, with a large black spot in the middle, below which is sometimes a smaller black spot. Sometimes there is a bright white line around the light area. The front upper part of the abdomen is yellowish gray, and the under part brown. The cephalothorax is orange brown, except a black spot between the eyes. The legs are orange brown.

Theridium differens.—Female about one-eighth of an inch long, and the male smaller. The abdomen is round, and the middle stripe often very brightly colored, with white or yellow at the edges and red in the middle [(fig. 264)]. The rest of the abdomen is reddish brown, darkest next to the white edge of the stripe. There are no distinct marks on the under side. In males the stripe on the abdomen is obscure, and the whole abdomen dark reddish brown [(fig. 265)]. Sometimes, especially in young spiders, the abdomen is entirely yellow, with indistinct brown markings. The cephalothorax is orange brown, often darker in the middle, but with no distinct stripe. The legs and palpi are yellow in females and orange brown in males, slightly darker at the ends of the joints. The epigynum has no openings in sight. They are on the inner side in the transverse fold across the abdomen. The palpal organ [(fig. 266)] has two appendages at the end, one hard and roughened and the other soft. The web is on low plants of all kinds, usually two or three feet from the ground [(fig. 263)]. There is sometimes a small tent, often hardly deep enough to cover the spider, from which the web spreads two or three inches, according to the shape of the plant. The cocoons of eggs are white and nearly as large as the spider, and are attached in the nest.

Theridium murarium.—Length about one-eighth of an inch, with the abdomen nearly spherical. The general color is gray. The legs are pale, with dark bands at the end and middle of each joint. The cephalothorax is pale, with a dark line in the middle and one on each side, the middle line sometimes divided into two near the eyes [(fig. 267)]. On the abdomen there is an undulated middle stripe, white at the edges and the front end, and reddish in the middle. On both sides of this stripe the abdomen is nearly black and becomes gradually lighter toward the sides. The sternum is pale, with a black edge and black stripe in the middle. The under side of the abdomen is gray, with a long black spot in the middle and a smaller one over the epigynum. There is little difference in size or color between the sexes. The epigynum [(fig. 269)] has two round holes, wide apart, near the thickened edge. The palpal organ [(fig. 268)] is shorter and simpler than it is in differens.

Theridium spirale.—This is a round-bodied spider of the same size as differens and murarium. The cephalothorax is orange brown above and below, with an indistinct dark stripe as wide in front as the eyes and narrowed behind. The abdomen has a middle stripe like differens, nearly as wide in front as it is in the middle [(fig. 271)]. The rest of the abdomen is gray, darkest toward the stripe. The legs are pale, sometimes with faint gray rings at the ends and middle of each joint. The middle stripe of the abdomen is sometimes reddish as in murarium, but oftener gray, with a dark spot near the front end. The males [(fig. 270)] have the same color and markings as the female and are sometimes more distinctly marked. The male palpi [(fig. 272)] are very large, and the palpal organ has a long tube coiled on the under and outer side. The openings of the epigynum [(fig. 273)] are about their diameter apart.

Theridium frondeum.—White, light yellow, or greenish white, with black markings that are very variable [(fig. 274)]. Usually the cephalothorax has two fine black lines running back from the eyes and uniting behind the dorsal groove, and black edges. The legs are usually darkened with brown at the ends of the joints. The abdomen is large and round, and has on the back a light undulated band bordered by brownish translucent spaces, with two black spots just over the spinnerets. Sometimes there are black spots in the translucent spaces, especially toward the hinder end, and these may be united into two long black stripes. In some individuals of either sex the black on the cephalothorax forms a wide band in the middle, almost covering the back, and a black stripe of similar width extends backwards on the abdomen for half its length. These black-striped individuals have all the other variations of color and markings. The males have all the colors and spots brighter and the legs longer than the females. The mandibles of the male are longer than those of the female and have at the base, in front, a low conical point.

This species is found from the White Mountains to Alabama. In New England it matures in July and is found on bushes all summer.

Theridium unimaculatum.—This little species differs in color and markings from all the others, and may almost always be distinguished by the white abdomen, with a black spot in the center of the back. The females are a twelfth of an inch long and the males smaller. The cephalothorax is orange yellow, with a black spot around the eyes, extending back in a point as far as the dorsal groove, and there is also a fine black line along the edges. The legs are orange, lighter in the female and darker in the male, with the first and second pairs in the male much stouter. The sternum is orange, with black edges. This spider makes a web, like the other small species, among small leaves and winters under dead leaves on the ground.