Casting a Sticky Trap: SPIDERS and Their Predatory Ways

Bennett C. Moulder, ISM Research Associate and Professor Emeritus, Illinois College, Jacksonville, Illinois

ating insects and other small sticky silk, capable of trapping and holding to the spot, and impales the insect with its arthropods is what spiders do for even relatively large an powerful insect prey. long chelicerae. First using her mouthparts a living. That spiders are an Damage to the orb caused by wind or strug- to cut a small slit in the tube, the spider enormously successful group of gling prey can be quickly repaired, or the then pulls the insect inside. After her meal, animals is testimony to how efficient they entire can be web taken down and replaced. the spider repairs the tear in the tube, re- areE at catching their prey. Most insects move Many orb weavers replace their tattered web sumes her position below ground, and swiftly and, for their size, are quite powerful. with a new one every day. awaits the next victim. “For spiders to capture such prey, they have, Other spiders do not utilize webs for prey Mastophora, the bolas spider, is a mem- as a group, developed an impressive arsenal capture. Crab spiders, perfectly camouflaged ber of the family Araneidae, the typical orb of weapons and a variety of strategies. against their background, lie in wait on flow- weavers. This small spider has abandoned Except for one small family (Ul- ers or on the bark of trees to ambush unsus- the practice of orb web construction alto- oboridae), all spiders in Illinois have venom pecting insects. Wolf spiders, fisher spiders, gether. Nonetheless, silk plays a prominent for killing or paralyzing their insect prey. and jumping spiders are keen-sighted hunt- role in its prey procurement. Mastophora is The spider seizes the insect using mouth- ers that actively search for their prey. a nocturnal hunter, spending the daylight parts called chelicerae and injects the venom Still other spiders exhibit rather bizarre hours clinging to a tree branch. In the though hollow cheliceral fangs. or unique methods of capturing prey. evening, she moves to the tip of a twig and Many spiders also rely heavily upon Among the spiders known to occur in Illi- fastens both ends of a silken line to the twig. silken snares or webs to entangle and ren- nois are Sphodros atlanticus der helpless various flying insects. Webs take (purse web spider), Mastophora many different forms, but all are constructed bisaccata (bolas spider), of a scleroprotein of remarkable strength and Mimetus puritanus (pirate spi- elasticity. Silk is produced as a liquid in spe- der), and Scytodes thoracica cial abdominal glands and is drawn out as (spitting spider). All of these threads from appendages called spinnerets spiders have quite interesting located at or near the end of the abdomen. prey procurement techniques. Of the many web forms, there are a few Rather formidable-looking, that are of particular interest in Illinois. The the purse web spider is related long-legged cellar spider (Pholcus) spins a to the large hairy tarantulas and loose, irregular web in corners of dark base- trapdoor spiders. She excavates ments or outhouses. The common house a long burrow in the soil at the spider (Achaearanea) spins a similar web. In base of a tree and lines the bur- addition to an irregular maze of threads, the row with silk. The silken lin- grass spider (Agelenopsis) makes a horizon- ing is extended up the side of tal platform with a funnel-like retreat at one the tree as a tube, or “purse,” end in which it hides, waiting for an insect for a distance of about ten to fall upon the platform. The crowning inches (fig. 1). The spider takes achievement of engineering in web con- her position inside the burrow struction is the familiar two-dimensional just below soil level. If an in- symmetrical orb web of the garden spider sect chances to walk across the (Argiope) and its many relatives. The typi- aerial part of the tube, the spi- cal orb web incorporates a close spiral of der senses the vibrations, rushes Figure 1. Purse web of Sphodros against a tree trunk

THE LIVING MUSEUM • VOLUME 59 • NOS. 1 & 2 • 1997 3 She then spins out a short thread that prey’s legs. Then a bite by the in- remains attached to her spinnerets, truder to the front leg of its victim combing out viscid material with her introduces a quick-acting venom. hind legs that accumulates as large Almost immediately, the bitten spi- drop of glue at the tip of the thread der succumbs and is then fed upon to form a casting line or bolas. Hang- by the pirate. The pirate spider’s ing by the hind legs of one side of strategy is a dangerous one, how- her body from the trapeze line at- ever, because occasionally its in- tached to the twig, she grasps the tended victim turns the tables and casting line by a front leg in antici- succeeds in making a meal of the pation of the approach of a night- pirate spider. flying moth (fig. 2). When a moth Perhaps the strangest method of comes near, the spider swings the prey capture by spiders is that exhib- bolas line so that the sticky tip strikes ited by Scytodes, the spitting spider. the body or wing of the victim. Ar- This is a small yellow spider with rested by the viscid, elastic thread, black spots. Scytodes may be found the moth struggles in vain to escape, in houses, where it comes out at and the spider quickly draws in her night to search for small insects. If a line and administers a venomous suitable prey animal comes within a bite. The moth is soon paralyzed and short distance of the spider (usually enswathed in sheets of silk, after a half inch or so), the spider’s body Figure 2: Bolas spider (Mastophora) which the bolas spider enjoys a lei- with casting line convulses and a jet of gummy mate- surely meal. One may wonder if the rial is emitted from each chelicera success of the bolas spider in captur- (fig. 4). As the twin streams of gum ing moths is attributable solely to its are ejected, the chelicerae oscillate patience in waiting for a moth to rapidly, causing the lines of gum to come near enough or if other forces are at work. Evidence strongly cover the victim’s body in a zigzag pattern. The insect is thus glued suggests that the spider produces a volatile chemical that serves as securely to the substrate, and the spider walks forward and admin- an olfactory attractor. The chemical probably mimics the sex phero- isters the coup de grace. After the victim’s struggles cease, it is disen- mone produced by female moths, since the prey of the bolas spider tangled from the gum and eaten. are always male moths. Scientific studies by many investigators reveal that spiders are Mimetus, the pirate spider, builds no web to snare prey. This present in all terrestrial communities in enormous numbers, and strange little spider is sluggish and has poor eyesight, seemingly ill the effect of their predatory activities must be very significant. Their equipped to capture prey without the aid of a web. Remarkably, predation techniques, ranging from the mundane to the bizarre, the pirate spider has adopted the practice of warily invading the have become highly specialized and efficient. Since spiders take all web of another spider with the aim of killing and devouring the sorts of insect prey—beneficial and pest species alike—it is debat- occupant. The spiders upon which Mimetus most often preys are able whether spiders on the whole are beneficial, harmful, or neu- orb weavers and comb-footed spiders tral from a human vantage point. In either (relatives of the black widow). Apparently, case, many people find that the patient once the pirate spider has stealthily en- study of the behaviors of these fascinat- tered its victim’s silken domain, it may lure ing creatures can be a source of constant the resident spider closer by shaking the amazement and delight. ❦ web to simulate a captured insect. The aroused occupant rushes to dispatch its Editor’s Note: For further reading see Bennett supposed prey, only to be met by the ne- Moulder’s book, A Guide to the Common farious Mimetus. A row of long spines on Figure 3. Mimetus front leg (metatarsus), Spiders of Illinois, Illinois State Museum each front leg (fig. 3) aids in grasping the showing spines Popular Science Volume X, 1992.

Figure 4. Spitting spider (Scytodes) and prey held down by twin jets of gum

4 THE LIVING MUSEUM • VOLUME 59 • NOS. 1 & 2 • 1997