Eight-leggedTricksters Spidersthat specializein catchingother spiders Robert R. Jackson Spiders arewell knownas preda- strugglesof a trappedinsect. It thereby tors of insects, but some even eat lures the residentspider close enough their own kind. For most spe- Use of deceit may be to be attacked (Bristowe 1941, cies, eating other spiders appears to be Gerhardt 1924). Czajka (1963) also largely an opportunistic occurrence, a especiallyimportant describedan instancewhere E. furcata larger or faster individual overpower- because another spideris a caught a female spider,apparently by ing another in a chance encounter. mimicking the courtship web vibra- There are at least a few spider species, potentialpredator as well tions of a male of the victim'sspecies. however, for which araneophagy (pre- More recently,Mimetus maculosus dation on other spiders) is routine as potential prey from Australiaand Mimetussp. from (e.g., Jackson and Poulsen 1990). Some New Zealand (Jackson and White- of these species employ strategies based house 1986) have beenshown to catch on deceit of their spider prey. In this Aggressive mimicry, where a preda- other spiders by using deceptive vi- article, I use the images of deception, tor seems beneficial to its prey, is a bratingsignals (Figure1). The deceiv- mimicry, and trickery to convey the ploy sometimes used by araneophagic ers ease onto webs, then transmitsig- functional significance of a predator's spiders. An aggressive mimic might, nals to their intended victims by behavior and not to imply cognition. for example, imitate an insect that is pulsatingtheir bodies up and down on Use of deceit by araneophagic spiders the intended victim's prey. In this the web and by pluckingand tugging may be especially important because way, an aggressive mimic can gain on the silk with their legs in a variety another spider is a potential predator control over its prey's behavior. The of ways, apparently mimicking as well as potential prey. situation becomes even more com- strugglesof small insects in the web. Approximately 34,000 species of plex when an araneo-phagic spider The mimetids illustrate several spiders have been described and interacts with another spider that is points that apply more generally to grouped into 105 families (Cod- both potential prey and potential aggressive mimic spiders. First, the dington and Levi 1991). Spiders are predator. Of the araneophagic spe- signalingbehavior of aggressivemim- often informally separated into two cies, the aggressive mimics are a small ics is complex. Mimetidsperform sev- behavioral groups: the web builders minority found only in five distantly eraldifferent vibratory behaviors, and, and the cursorial spiders (Foelix 1982). related families. by varyingthe sequencein which they Web builders erect silken webs and performthese behaviorsand the char- wait for their prey to arrive. In con- Pirate spiders as acteristicsof each behavior(e.g., am- and of trast, cursorial species ambush or stalk aggressive mimics plitude speed leg movement), and chase down their prey without they can transmita complex arrayof using webs. But araneophagic species Although pirate spiders (Mimetidae) vibrations across the web to the host tend to be behavioral nonconform- are well known as web-invading spider (Jackson and Whitehouse ists; they fail to fit simply into either of araneophagic spiders (Foelix 1982), 1986). these traditional behavioral groups. for only a few species from this cos- Second, the basic principles that mopolitan family are details known govern intraspecific communication about behavior. Ero furcata, a Euro- also govern interactionsbetween spe- Robert R. is an associate pean mimetid, is a frequently cited cies duringaggressive mimicry. When Jackson profes- A sor in the Zoology Department,Univer- example of an aggressive mimic. animalscommunicate, one individual sity of Canterbury, Christchurch,New member of this species moves onto (the sender) indirectly manipulates Zealand. ? 1992 American Institute of other spiders' webs, where it jerks on another individual (the receiver) by Biological Sciences. the web, apparently mimicking the providing a specializedstimulus (the 590 BioScience Vol. 42 No. 8 This content downloaded from 185.2.32.89 on Fri, 20 Jun 2014 00:31:39 AM All use subject to JSTOR Terms and Conditions signal)to which the receiverresponds (see Dawkins and Krebs 1978). In aggressive mimicry systems of spi- ders, a predatorprovides another spi- r: der species,its prey,with a signal. But _W in this instance,the predatorattempts to elicit a responsethat benefits itself r-....~ ~~.. I and not the prey. Whetheror not the receiverbenefits in a communication system is an important question be- .r. IP cause the evolution of the receiver's / behavior is easy to account for if it I~~ benefits, but more difficult if it does I~~~ not. The function of the signal, how- ; ever, refers only to how the signal benefits the sender. Third, mimetids, like all web-in- vading aggressivemimic spiders that have been studied in detail, are not limited to using aggressive mimicry (Jacksonand Whitehouse 1986). In- stead, they are versatile predators (Curio1976) whose tacticsdepend on the circumstances (see Dominey Figure1. Mimetusmaculosus (above, facing down, ventral view), having attacked and 1984), and so the tactics are consid- killedanother spider, Eriophora pustulosa (below, facing right, ventral view), wraps eredto be conditionalpredatory strat- its victimup in silk. egies. Mimetids, in addition to catch- ing the spidersin their own webs, also steal insects from other spiders'webs ever,adhere to cribellatewebs and are pholcid that buildsa nonstickydome- (klep-toparasitism)and open egg sacs less efficient as predatorsthere. Some shaped web and, like other pholcids, to feed on the victim spider's eggs other aggressivemimics are more ef- has legs that are especially long rela- (oophagy). These behaviors are dis- fective when invadingcribellate webs tive to the body. When prey contacts tinctly different from araneophagic than when invadingecribellate webs. the web, P. phalangioidesuses its long aggressivemimicry because no signal- How mimetidsand other web-invad- legs to wrap the prey up with silk ing is associated with them. ing spiders avoid adhering to sticky while keepingits body distantand out Fourth, the characteristicsof the webs is not known. of harm'sway. invaded web largely determine how Except for salticids, which will be However, P. phalangioides is not effective an aggressive mimic is at discussed later, spiders do not have simply a predator that uses its own catching the web's resident. Spider acute vision. Typical web-building web. Like Mimetus, P. phalangioides webs varygreatly in shapeand design, spiders rely primarily on web-borne invades alien webs and practices ag- from two-dimensional orb webs to vibrationalcues to detect and locate gressive mimicry and also eats the sparselyspun three-dimensional space prey. A web invader, also without resident spider, insects in the alien webs to thickly woven sheet webs acute vision, relieson vibrationalcues web, andthe victimspider's eggs (Jack- (Foelix 1982). Some spiders, called in the alien web. But because webs son and Brassington1987). cribellates,build sticky webs by coat- may differsignificantly in their vibra- Unlike Mimetus, P. phalangioides ing structurallines with fine threads tional characteristicsdue to structural adheres to both cribellate and fromthe cribellum,a specializedspin- diversity,web invadersare limited as ecribellatesticky webs. Therefore,P. ning plate with minute spigots. Other to the range of web types they can phalangioidesis moreefficient at catch- spiders,called ecribellates, build sticky exploit. This sensory factor appar- ing prey on nonsticky than on sticky webs by secretingfrom spigots on the ently limits most aggressivemimics to webs, and most efficient of all when spinneretsadhesive fluid to form glue being effective as predators on a re- on its own web (Jackson and dropletsalong the threads.Webs with- stricted range of web types (Jackson Brassington 1987). Nevertheless, P. out eitherform of glue are referredto 1986). phalangioidescan catch prey on both as nonsticky, although all spider silk types of sticky webs. With its long maybe adhesiveto someextent (Hallas Both a web builder and a legs, P. phalangioidesappears to tip- and Jackson 1986). web invader toe across the sticky web, minimizing Mimetids are most effective when contact with the sticky threads (Fig- invadingthe space and orb webs cre- Research on Pholcus phalangioides ure 2). Also, by grooming its legs ated by ecribellates (Jackson and (Jacksonand Brassington1987) illus- frequently, P. phalangioides can re- Whitehouse 1986); they do not ad- tratesthat an individualspider may be move any sticky silk that accumu- here to the glue on these most fre- both a web builderand a web invader. lates.Another key to P. phalangioides' quentlyinvaded webs. They do, how- P. phalangioides is a cosmopolitan success at invading sticky webs ap- September1992 591 This content downloaded from 185.2.32.89 on Fri, 20 Jun 2014 00:31:39 AM All use subject to JSTOR Terms and Conditions a.__ R - If Zealand illustrates that an ..;,. X. _ gnaphosid, can * araneophagic web-invading spider be both a web builder and a cursorial * *".. predator. Most gnaphosids are insec- "*' -1 e ;F . X* \jh'. \ tivorous hunters. T. erebus, like typi- _0 - ow.,. Oe1\ \\ cal gnaphosids, generally hunts prey f in the open, completely away from 4.-leX4 webs. However, an