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Natural history of the pirate spider Mimetus hesperus (Araneae; Mimetidae) in Kern County, California.

Pirate spiders (Mimetidae) are known for their unusual preferences of prey and predatory tactics. Their diet consists mostly of other spiders, which they capture primarily by invading webs and using a vibratory lure to bring the resident within range of their deadly strike (Archer, 1941; Czajka, 1963; Cutler, 1972; Lawler, 1972; Lowrie et al., 1972; Jackson and Whitehouse, 1986; Kloock, 2000, 2001). Although Jackson and Whitehouse (1986) and Kloock (2000, 2001) have made contributions, we know little about natural history of these spiders. This is especially true for the pirate spider Mimetus hesperus, which occurs throughout the southern united States and Mexico (Mott, 1989). In the western part of its range, M. hesperus occurs northward through Washington, Idaho, and British Columbia and has been identified as far east as Cuba (Mott, 1989).

Mimetus hesperus appears to have rather broad habitat preferences and has been reported in agricultural fields such as cotton (Dean et al., 1982) and peanuts (Agnew et al., 1985) in Texas, grapes in California (Costello and Daane, 1998), and mangos and citrus in Mexico (Llinas-Gutierrez and Jimenez, 2004). Only Llinas-Gutierrez and Jimenez (2004) have reported M. hesperus in a nonagricultural plant (mesquite Prosopis). other authors have defined generally broad habitats (e.g., pinyon-juniper Pinus-Juniperus woodland; Muma, 1980; R. Delph et al., in litt.) but not individual species upon which M. hesperus occurs.

In 2007, I discovered a M. hesperus living in a mixed stand of saltbushes (Atriplex) in Kern County, California. During the next 2 years, I located several more individuals at several sites in and around Bakersfield, California, all in saltbush communities. in 2010, I conducted systematic surveys in saltbushes at the Panorama Vista Preserve in Bakersfield to study natural history of this spider. This paper documents seasonal abundance of M. hesperus in the preserve and interactions between M. hesperus and potential prey spiders.

MATERIALS AND METHODS--During April-November 2010 (1 week was missed in July), I conducted weekly, nondestructive, visual censuses of the nocturnally active community of spiders on saltbushes at Panorama Vista Preserve, Kern County, California. This habitat contained mixed patches of saltbushes including Atriplex lentiformis, A. polycarpa, A. serenana, and nonnative A. semibaccata (Australian; California Invasive Plant Inventory, 2006). No attempt was made to distinguish species of Atriplex. All spiders identified were on Atriplex or in webs attached to Atriplex, although a variety of grasses grew in and around the patches. in April 2010, I conducted a series of four weekly censuses to determine feasibility of different techniques. Based on these trials, I developed methods used during May-November, and only those data, using consistent methods, are reported here.

I conducted censuses with the aid of a headlamp and magnifying eyeglasses (+4 magnification). On each night of censusing, I randomly established six transects of 5 m each within patches of saltbushes. I chose 5-m transects because, in trials, this proved to be a convenient size given that size of patches of saltbushes in the preserve were <2 to >50 m in circumference. Only patches with circumferences >10 m were sampled. I established each transect by randomly choosing a starting point and laying out a 5-m-long measuring tape along the edge of a patch of saltbushes, loosely following curves of the patch.

I identified (usually to genus) and counted all spiders readily visible without extensive movement of vegetation, which would have disturbed neighboring spiders. No overlap between transects occurred during any night of censusing. Distances between transects varied, but transects were never closer than 10 m and always were in noncontiguous patches on any given census. New transects were measured each week and it is likely that areas of overlap occurred during the study. I also recorded the time at which each M. hesperus was found. I determined times of sunset using tables obtained from the United States Naval Oceanography ( astronomical-applications/data-services/rs-one-year-us), and all times are expressed in minutes after sunset to account for seasonal variation and daylight-saving time. Censuses typically began near sunset and lasted, on average, until 264 min after sunset (range, 119-405 min).

I generally identified spiders only to genus for two main reasons. First, because I sampled throughout the entire season, many spiders were juveniles, which often are difficult to identify to species. Second, because I used repeated sampling, collecting spiders for identification was not desirable because of the potential impact on the community of spiders. Of course, I identified M. hesperus, the only mimetid in the study area, to species. Small juvenile spiders often could not be identified in the field and were labeled as unknown. Occasionally spiders leapt from webs or vegetation before identification could be made; these were either labeled as unknown or identified to family. Families that occurred infrequently generally were identified only to family. Representative individuals of unrecognized spiders were collected, brought to the laboratory, and identified using Ubick et al. (2005).

I define an interaction as any situation in which [greater than or equal to] 2 spiders occurred in the same web or were in physical contact. Interactions between M. hesperus and other spiders were recorded and observed until one of the spiders left the web or was disturbed by my observation. Prey spiders were collected and brought to the laboratory for identification.

RESULTS--I found 5,772 spiders representing 13 families (Table 1), 40 of which (0.7%) were M. hesperus. Of the 40 M. hesperus, four were adult males and five were adult females. The remaining 31 were juveniles. On average, one M. hesperus was found for every 144 spiders located (all Mimetus per all spiders including roaming spiders), and one M. hesperus/111 occupied webs (all Mimetus per occupied webs).

Table 2 summarizes interactions observed between M. hesperus and other spiders. The two M. hesperus found with Holocnemus were actually on the same web in the middle of a complex of several webs joined together, including webs of Holocnemus and Uloborus.

Mimetus hesperus occurred throughout the evenings (average = 178 min after sunset, SD = 81.3 min, range = 36-312 min). Fig. 1 depicts seasonal abundance and distribution of adults and juveniles of M. hesperus. Of adults, 33% (three of nine) were located on 12 August 2010, but, as shown in Fig. 1, adults occurred intermittently during May-September.

DISCUSSION--Given that mimetids feed high on the food chain, it is not surprising that M. hesperus occurs in low density. Previous studies reported an average relative density of one M. hesperus/193 spiders (Branson, 1966; Muma, 1975, 1980; Costello and Daane, 1998; Llinas-Gutierrez and Jimenez, 2004; Duran-Barron et al., 2009). Relative density measured by me (one M. hesperus/144 spiders) seems likely to be typical, because other studies did not specifically target M. hesperus and, thus, were likely to be underestimates. Mimetus hesperus appears to exist at a lower per-web density (1/111 webs) than did Mimetus notius, for which Kloock (2000) reported an average relative density of one M. notius/25 webs across 19 sites in southwestern Michigan.

Kloock (2001) determined that the most abundant prey for M. notius were insects (16% of diet), Theridion (16%), and Dictyna (8%). The spider portion of the diet of M. hesperus appears similar to that of M. notius (Table 2) but with Dictyna being more common than Theridion. This reflects relative abundances of prey, because, in Michigan, Theridion was more abundant than Dictyna (Kloock, 2000), whereas, in California, the reverse was true (Table 1).

Uloborus experiences invasions of webs from M. notius (2% of invasions; Kloock, 2001) and M. hesperus (Table 2) but without successful predation observed in either instance. Kloock (2001) discovered that M. notius kleptoparasitizes insect prey in webs of spiders whose webs it invades but does not prey upon, including Uloborus. I observed no insectivory in M. hesperus, and it is possible that inclusion of insects in the diet partly explains the difference in relative density between these species. However, given the small number of predatory events (n = 4) observed, insectivory and kleptoparasitism in M. hesperus cannot be ruled out.

Jackson and Whitehouse (1986) reported that Mimetidae in New Zealand experienced highest predatory success on spiders from the families Theridiidae and Araneidae and were unable to prey on Pholcidae. Kloock (2000, 2001) noted a similar pattern for M. notius, as well as predation on Dictynidae, which Jackson and Whitehouse (1986) did not test. Mimetus hesperus, like M. notius, successfully preys upon Theridiidae and Dictynidae and invades webs of pholcids with no observed success (again with the caveat of small samples). The fact that some pholcids, including Pholcus phalangoides, also have been identified as araneophagic invaders of webs (Jackson and Brassington, 1987;Japyassu and Macagnan, 2004) maybe significant in the apparent lack of success of mimetids against this group. Mimetus hesperus has not been observed invading webs of araneids, but araneids occurred in low density in my field site (Table 1), so this may result from a difference in availability rather than ability. Mimetus hesperus also has been identified at nearby sites with more dense populations of araneids, and future work will investigate the potential predatory relationship between M. hesperus and araneid spiders.

Kloock (2000) detected strong seasonality with little overlap between generations in M. notius. Mimetus notius over wintered as adults, bred in spring, and most adults died before juveniles emerged from eggs, resulting in a sharp decline in populations during midsummer. Kloock (2000) noted the same general pattern in Theridion, their primary spider prey. Mimetus hesperus and its prey exhibited weak seasonality at best (Fig. 1), and adults and juveniles occurred throughout the spring-autumn activity season. The much stronger seasonality of climate in Michigan compared to climate in California most likely accounted for this difference.

Czajka (1963) suggested that the mimeted Ero furcata mimicked mating signals of its prey to lure them within range. This seems unlikely for M. notius because juveniles represented a large portion of prey, particularly following the mid-summer decline in abundance when adults generally were unavailable (Kloock, 2000). However, three of the four successful captures of prey by M. hesperus were of adult males (Table 2). Again, because of the small sample, firm conclusions are impossible, but this combined with the lack of strong seasonality in this population make it plausible that M. hesperus uses mimicry of mating signals as a vibratory lure.

Overall, M. hesperus appears to be ecologically similar to other mimetids that have been studied, but several differences, particularly in relative density of populations and seasonality, exist. A better understanding of its ecology will require further study. To investigate predatory success against different potential prey spiders, staging predatory opportunities in the laboratory following Jackson and Whitehouse (1986) will be necessary.


I acknowledge A. Honig, S. Honig, and C. Belli of the Panorama Vista Preserve for allowing access to the preserve, and D. Zuniga and Y. Missbach for aid in collecting and organizing data. A. Castaneda and M. Piris aided with translation of the abstract. I also thank two anonymous reviewers whose comments greatly improved the manuscript.


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Submitted 10 July 2011. Accepted 11 May 2012.

Associate Editor was Jerry L. Cook.


Department of Biology, California State University, 9001 Stockdale Highway, Bakersfield, CA 93311

TABLE 1--Spiders (Araneae) identified in saltbushes (Atriplex)
at the Panorama Vista Preserve, Kern County, California, May-November
2010. An asterisk indicates interactions with the
pirate spider Mimetus hesperus were observed.

Taxon                       observed

Agelenidae                     19
Araneidae                      33
Dictynidae (Dictyna) *      2,821
Gnaphosidae                     1
Lycosidae                       6
Mimetidae (M. hesperus)        40
Oecobiidae                      1
Philodromidae                 869
  Ebo                         849
  Philodromous                 20
Pholcidae *                    37
  Holocnemus *                  3
  Pholcus                      16
  Unidentified                 18
Salticidae                    128
Theridiidae*                  836
  Euryopis                      2
  Latrodectus                   7
  Theridion*                  827
Thomisidae                      9
Uloboridae (Uloborus) *       108
Unknown (including            864
spiderlings and escapees)

Total number of spiders     5,772

TABLE 2--Summary of pirate spiders (Mimetus hesperus) found on webs
and with prey during censuses at the Panorama Vista Preserve, Kern
County, California (40 M. hesperus were observed). Without a
resident spider, webs of Dictyna and Theridion were not reliably
identifiable, so these were recorded as branchtip. Similarly,
pholcid indicates a vacant web typical of the family Pholcidae. The
four feeding M. hesperus were outside of webs.

Number of M.    Number of M. hesperus    Number of M. hesperus
hesperus on     with live resident in    feeding (type of prey)
vacant webs     web (type)

3 (branchtip)   3 (Dictyna)              1 Juvenile (Dictyna)
1 (pholcid)     2 (Holocnemus)           2 Adult male (Dictyna)
1 (Uloborus)    1 (Uloborus)             1 Adult male (Theridion)
5 (12.5% of     6 (15.0% of total)       4 (10.0% of total)
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Author:Kloock, Carl T.
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1USA
Date:Dec 1, 2012
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