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How Spiders Make a Living Fonuu How Spiders Make a Living M. NYFFELER, W. L. STERLING, aNo D. A. DEAN Department of Entomology, Texas A&M University, College Station, TX77843 Environ. Entomol. 23(6): 1357-1367 (1994) ABSTRACT Although the beneffcial status of the spiders as insectivores has been widely recognized for quite some time, biologists by and large seem to be rather unfamiliar with the speciffc feeding habits of this very diverse order. We present an overview of the feeding patterns of 10 groups of common agroecosystem spiders to inform entomologists and ecologists concerned with issues of natural biological control. The various spider groups discussed in this article exhibit a very diverse range of life styles and foraging modes, which is reflected in the diversity of their feeding patterns. Implications of the insectivorous activities ofthese predators for natural pest control are discussed. KEY WORDS spiders, predation, diets Sprorns (AneNner) ARE a very diverse order of ingest solid food and must, therefore, inject di- ubiquitous carnivores within the class Arach- gestive enzymes into the immobilized prey (ex- nida. At the present time, >30,000 species of ternal digestion) and then suck in the dissolved spiders are described (Coddington & Levi 1991). tissue in liquid form. Spiders generally have a Over 3,000 species occur in North America alone very low rate of metabolism compared with other (Young & Edwards 1990). The vast majority of poikilothermic organisms of comparable body spiders occupy terrestrial habitats. Some ly- weight (Greenstone & Bennett 1980). They can cosids and pisaurids, however, can walk and sail store energy and starve for considerable time on the water surface (and at times even dive and periods, which makes them excellent survivors swim under water); they forage on aquatic and under conditions of food shortage (see Nyffeler & semiaquatic organisms when they inhabit Breene I990a). marshes, flooded rice fields, and other wetlands According to traditional foraging theory, spi- (Greenstone 1979, Oraze & Grigarick 1989, Zim- ders are considered to be predators of live, mov- merrnann & Spence 1989). One agelenid species ing prey only (e.g., Turnbull 1960, 1973). More (the water spider) actually lives under water (see recent studies have modiffed this view when ev- subsection Foraging Patterns of Web Weaoers). idence was found that spiders utilize a much Most spiders are highly cannibalistic solitary broader range of foraging strategies, including creatures and practice bizarre courtship rituals feeding on arthropod eggs (oophagy), dead ani- (Tumbull 1973). Several species produce sounds mals (scavenging), plant pollen, and even ar- (acoustic communication) during courtship and tiffcial diets (see McDaniel & Sterling 1982, agonistic displays (Rovner 1975,Uetz & Stratton Nyffeler et al. 1990a). Stealing ofprey from other 1982). These animals live in a world full of vi- spiders (kleptoparasitism) plays an important brations (e.g., Rovner & Barth l98l). Sexual di- role as an alternative foraging strategy ofvarious morphism occurs in many species, the female web spinners (Vollrath f987). Spiders have been normally being significantly larger than the male reported feeding on a wide range of different (hereafter adult length always refers to the fe- animal groups including some unusual prey such male). Spiders disperse by walking on the as small mice, bats, birds, ffsh, crayfish, crabs, ground, by using silk-thread bridges between frogs, lizards, snakes, and scorpions (Nyffeler & plants, as well as ballooning through the atmo- Benz 1981, McCormick & Polis 1982); however, sphere from place to place on silken threads in general they tend to concentrate on insect (Foelix 1982, Dean & Sterling 1985, Young & prey and to a lesser degree on other spiders Edwards 1990). All spiders produce silk from (Wise 1993). Most spider species forage on mul- abdominal glands though only the useb weaoers tiple prey species (generalist predators), which construct webs that are used to catch prey. Spi- Greenstone (1979) has suggested may be advan- ders are equipped with a pair ofjaws (chelicerae) tageous by optimizing a balanced essential and possess venom glands (exception, Ul- amino acid composition in the diet. Spiders feed oboridae do not produce venom). Immobiliza- predominantly on small-sized prey relative to tion of prey is assisted by the use of silk and by their own size (prey length s predator length) the injection of venom. These animals cannot (Nyffeler & Benz 1981, Wise 1993); feeding ex- OO46-225X/941 1357-1367$02.00/0 @ 1994 Entomological Socie9 of America This article is the copyright property of the Entomological Society of America and may not be used for aqy commercial or other private purpose without specific r,rnitten permission of the Entomological Society of America r358 ENvrnoNurrvrer, EurouolocY Vol.23, no.6 periments with a variety of spider species and a Swiss Federal Institute of Technology, respec- model prey (crickets) conducted in the labora- tively. tory revealed that the optimal prey length ranges from 50-80Vo of the spider length (Nentwig re87). Results and Discussion Spiders are among the numerically dominant Foraging Patterns of Web Weavers. High feed- insectivores in terrestrial ecosystems and exhibit ing frequencies (up to 90Vo spiders feeding si- a vety diverse range of life styles and foraging multaneously during peak activity) were ob- behaviors (Turnbull 1973, Wise 1993). Two basic served in ffeld populations ofcertain larger-sized groups of foraging strategies can be distin- orb weavers (Araneidae) that rebuild (recycle) guished: (I) web spöders (i.e., foraging with a their webs daily (Nyffeler 1982). The high feed- catching web) (Tables l-5), and (2) hunters or ing frequencies indicate that the web is a very wanderers (i.e., foraging without the use of a efficient prey capturing device. Large orb weav- web) (Tables 6-9). Some prominent representa- ers often kill prey in excess of their energy re- tives of web spinning spiders are orb weavers quirements. As many as 1,000 small insects have (Araneidae and Tetragnathidae), sheet web been found entangled at one time in a single orb weavers (Linyphiidae), mesh web weavers (Dic- web; however, not all insects caught by the web tynidae), comb-footed spiders (Theridiidae), and are eaten. Sheet-web weavers, mesh web weav- funnel-web weavers (Agelenidae). Prominent ers, comb-footed spiders, and funnel-web weav- representatives of hunters are wolf spiders (Ly- ers that do not renew their nets daily, feed cosidae), lynx spiders (Oxyopidae), crab spiders less frequently (<IlVo spiders feeding simul- (Thomisidae), and jumping spiders (Salticidae). taneously) (M.N. & Benz 1988a; unpublished These I0 families are among the most abundant data). Relatively low feeding frequencies were spider predators in agroecosystems (e.g., Whit- also observed in small orb weavers that spin comb 1974, Luczak 1979, Nyffeler 1982, Dean & small delicate nets (LeSar & Unzicker 1978, Sterling 1987); and because oftheir high coloni- Culin & Yeargan 1982, Nyffeler 1982). The de- zation power and insectivorous feeding behav- signs and functions of different types of spider ior, they are of interest to the entomologist and webs are discussed explicitly by Eberhard (1990). ecologist concerned with issues of natural bio- Orb Weaoers. Orb weavers (Araneidae and logical control (compare Turnbull 1973, Riechert Tetragnathidae) spin spiraling sticky webs on & Lockley 1984, Nyffeler & Benz 1987, Sterling and between plants in a wide variety of ffeld et al. 1989). In this article, we present an over- crops and natural habitats. Many orb weavers view ofthe feeding patterns ofthese 10 groups of spin their webs preferentially at the beginning or spider predators. end of the nocturnal period (Foelix f982). Orb weavers wait in a head-down position for prey in the web center (hub) a Materials and Methods or in retreat connected to the hub by a signal line. Alerted by the vibra- There are different methods to evaluate spider tions of an insect struggling in the web, the spi- diets. The prey spectra of spiders can be as- der rushes to its victim; subsequently, the prey is sessed by directly collecting prey organisms or wrapped in silk followed by a venomous bite (in their remains from spider webs (i.e., prey analy- some cases, the prey is first bitten and then ses of web weavers) (Tables l-5), or collecting wrapped) (Foelix 1982). The immobilized prey is spiders with prey in their chelicerae in the ffeld later carried to the hub or retreat where it is (i.e., prey analyses of hunters) (Tables 6-9). Spi- eaten. T etra gnatha lab orio s a }{entz (Tetragnath- der predators (along with their prey) are placed idae), a slender elongate orb weaver (-6 mm in 70Vo ethyl alcohol and later identiffed in the adult length) of yellowish color with a silvery laboratory, using a dissecting microscope (see abdomen, is one of the most abundant spider Nyffeler et al. [987b, 1989] for details). Addi- predators of field crops in the United States tionally, sophisticated methods (e.g., release of (Young & Edwards 1990). With their fragile prey radiolabeled with 32P, ELISA techniques, webs (-lg-15 cm in diameter) oriented at vari- chromatography) are used to detect feeding on ous angles, these spiders trap small soft-bodied insect eggs, tiny aphids, and mites, and other insects predominantly of the orders Diptera and hidden predation activities that may otherwise Homoptera (Table l). Lea{hoppers (Cicadell- be overlooked with visual observation methods idae) represented an essential component (>30Vo (Greenstone 1979, McDaniel & Sterling 1982, of total) in the prey of T. laboröoso in soybean Nyffeler et al. 1990a). The prey spectra pre- ffelds in Illinois and Kentucky (Table l) (LeSar sented in this article (Tables l-9) are all based & Unzicker 1978, Culin & Yeargan 1982). In a on observational data from field studies previ- cotton ffeld in Texas, the prey of this species was ously published in literature (see references in composed largely of aphids (75Vo of total) (Table tables); a large portion of this information had l; Nyffeler et al.
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