132 NOTES

Caribbean Journal of Science, Vol. 44, No. 1, 132-135, 2008 Copyright 2008 College of Arts and Sciences University of Puerto Rico, Mayagu¨ ez

Observations of the Arachnocoris trinitatis (: ) as an inquiline of the Mesabolivar aurantiacus (Araneae: )

JO-ANNE N. SEWLAL1 AND CHRISTOPHER K. STARR2 Department of Life Sciences, Univer- sity of the West Indies, St Augustine, Trinidad & Tobago. Corresponding author: [email protected]

ABSTRACT.—The pholcid spider Mesabolivar au- rantiacus (Mello-Leitão 1930) is common in forests of Trinidad, West Indies. Its webs are often found to contain the nabid bug Arachnocoris trinitatis (Ber- groth 1916). In a lowland forest in August-September 2003 (wet season) we censused 81 M. aurantiacus webs for occupancy by the spider and the insect. A. trinitatis showed no significant preference for webs occupied by either juvenile or adult . How- ever, it showed a preference for empty webs, sug- gesting that it utilizes these as a ready-made prey- capture device, and possibly as a site for finding mates.

KEY WORDS.—Arachnocoris trinitatis, Nabidae, Me- sabolivar aurantiacus, Pholcidae, predation, symbio- sis NOTES 133

Mesabolivar aurantiacus (Mello-Leitão leaving non-commercial species to preserve 1930) is a web-building spider of the family the canopy. Five years after its inception, Pholcidae, widespread in the northern neo- this system was further modified so that tropics (Huber 2000). It is common in low- natural regeneration replaced artificial re- land and middle-level forest in Trinidad, generation. Timber harvesting ceased West Indies, where it is most often found in about 20 years before this study, and the the spaces between adjacent buttress roots area is now a seasonal evergreen forest of trees. Arachnocoris (Scott 1881) is a small (Beard 1946). Currently, the Arena Forest neotropical genus of the family Nabidae Reserve is used for watershed protection (). These are often found and a variety of recreational and minor eco- in spider webs (Kerzhner 1990), mostly of nomic purposes. Average monthly tem- the family Pholcidae. Coexistence between perature varies from 18.8°C to 28.3°C, with this nabid genus and pholcid spiders has recorded annual rainfall between 203.2 cm previously been reported from Costa Rica and 304.8 cm (Bell 1980). The ground is (Kerzhner 1990) and Guadeloupe (Lopez mostly gently undulating, varying in eleva- 1990). It has been assumed that A. triniatus tion from about 22 to 88 m (Bell 1980). is a kleptoparasite, feeding on prey caught We recorded the incidence of A. trinitatis in the webs (Harris 1928). in webs, their general position (middle or Arachnocoris spp. show physical adapta- periphery of the sheeting or on supporting tions for life in spider webs. These were threads), and their position relative to any investigated in detail by Myers (1925) for A. M. aurantiacus in the web, as well as the albomaculatus (Scott 1881). Their body co- number and ages (juvenile or adult) of resi- louration consists of white markings on a dent M. aurantiacus. dark ground colour, rather similar to the We gently agitated the sheeting of some kleptoparasitic spider genus (Si- webs with a dry twig in order to mimic mon 1864) and to some extent some genera ensnared prey, noting the reactions of A. of orb-weaving that spend extended peri- trinitatis and M. aurantiacus to the distur- ods of time suspended in the centre of their bance. A. trinitatis was also directly dis- webs, e.g. example Cyclosa (Menge 1866), turbed by touching with a dry twig to Nephila (Leach 1815) and Argiope (Audouin simulate contact by M. aurantiacus, some- 1826). times with attempts to nudge it toward any Webs of the pholcid Mesabolivar aurantia- M. aurantiacus in the web. This also served cus (Mello-Leitão 1930) in Trinidad, West to indicate the maturity of the nabids, since Indies consist of a sticky sheet web of hap- adults fly readily and juveniles not at all. hazardly organized irregular polygons Arachnocoris trinitatis can be found year- (Sewlal 2005). A tangle of non-sticky web- round in M. aurantiacus webs at the study bing above the sheet raises it to form a site, but in our experience it is much less dome. Juvenile and adult spiders build common in the dry season. In August and webs utilizing the same design. The pres- September 2003 (wet season) we examined ence of the nabid Arachnocoris trinitatis 81 M. aurantiacus webs. Of these, 20 webs (Bergroth 1916) in the webs of this spider contained at least one A. trinitatis (Table 1). raises the question of the relationship be- In each of 12 webs we found one A. trini- tween the two species. If A. trinitatis feeds tatis, in each of seven webs we found two, on prey trapped in the host spider’s web, is while the remaining web had three. One it kleptoparasitic or simply commensal? pair of A. trinitatis comprised an adult and We approached this question through ob- a juvenile, while all other conspecifically servations of the incidence and position of associated individuals were adults. In the A. trinitatis in M. aurantiacus webs. six adult pairs the individuals were ap- This study was carried out in the Arena proximately 0.5 to 4 cm apart from each Forest Reserve, Trinidad, West Indies other. In the adult-juvenile pair, the juve- (10°34ЈN61°14ЈW). This reserve initially nile remained mostly at the periphery of utilised the shelterwood system, in which the web, about 6 cm from the adult. species of commercial value are harvested, Of the 29 A. trinitatis found in webs, 17 134 NOTES

TABLE 1. Incidence of Arachnocoris trinitatis in webs of Mesabolivar aurantiacus in Trinidad, West Indies.

Occupancy of Mesabolivar webs No Juvenile Single adult Two adult Mesabolivar Mesabolivar Mesabolivar Mesabolivar Total Arachnocoris absent 9 25 23 4 61 Arachnocoris present 9 8 2 1 20 Total 18 33 25 5 81 were the sheet, 10 in the tangle, and the that were agitated to mimic struggling positions of the remaining two were not re- prey. Our observation of A. trinitatis attack- corded. In terms of the relative proximity of ing prey caught in the web and retreating at A. trinitatis to M. aurantiacus in occupied the approach of M. aurantiacus suggest a webs, 17 out 19 A. trinitatis were found in kleptoparasitic habit. Besides acting as a the tangle or, if in the sheeting, on its pe- prey-capture device, the web can also riphery, while M. aurantiacus was in the shield A. trinitatis from predators and M. middle of the sheeting. One spider was aurantiacus. The non-sticky tangle threads near the middle of the sheeting of its web, appear to act as a retreat for the A. trinitatis. and the location of the remaining pholcid This is seen in observations of M. aurantia- on its web was not recorded. The locations cus that did not retreat to the tangle but to of the remaining two nabids were not re- other parts of the sheet when disturbed. corded. Thus, the web architecture of M. aurantiacus It is harder to be certain of finding all A. webs evidently facilitates both prey capture trinitatis in a web than it is of finding all M. and protection from predators for A. trini- aurantiacus. Accordingly, for statistical pur- tatis. In addition, we think it is likely that A. poses we record A. trinitatis simply as ei- trinitatis treat webs as a favoured venue to ther present or absent. They showed no sig- seek mates. This latter hypothesis is sup- nificant preference for the webs of either ported by the observation that six out of adult or juvenile spiders (␹2 = 1.33; df = 1; seven (86%) of the pairs of A. trinitatis p = 0.29). However, there is a decided pref- found sharing a web each comprised one erence for webs without M. aurantiacus adult of each sex. present (␹2 = 6.32; df = 1; p = 0.01). A. trinitatis’s preference for vacant webs A. trinitatis was often observed either strongly suggests that the nabids depend walking or stationary on threads of the un- not on the pholcid but on its web as a derside of the web. When disturbed while means of catching prey. In occupied webs, in the sheet, the nabid retreated to the M. aurantiacus appeared to be treated with tangle or a supporting thread or flew out of caution, as seen in the distance that A. trini- the web. A. trinitatis also did not move to- tatis keeps from the spider, as well as the wards M. aurantiacus when we tried to one observation of the nabid retreating at stimulate them to do so. the approach of a spider. In contrast, when disturbed while in the tangle, the nabid would retreat to an indi- Acknowledgements.—We thank Bernhard vidual supporting thread and shake it. On Huber for identifying Mesabolivar aurantia- one occasion, while in a web with M. au- cus, Tom Henry for identifying Arachnocoris rantiacus, a nabid attacked prey that be- trinitatis, Izya Kerzhner for advice and Bill came entangled, but it then retreated when Eberhard, Jerry Rovner and an anonymous M. aurantiacus approached the prey. reviewer for suggesting improvements in Observations during this study show the paper. that A. trinitatis can utilize the sheet of sticky silk of a M. aurantiacus web as a prey- REFERENCES capture device. This was seen indirectly Bell, T. I. W. 1980. Arena Forest Reserve Working Plan when nabids moved to areas of the sheet 1971-1980. Forestry Research Inventory and Man- NOTES 135

agement, Forestry Department, Ministry of Agri- Lopez, A. 1990. Arachnocoris karukerae new-species He- culture, Trinidad and Tobago. miptera Heteroptera a commensal nabid of the Beard, J. S. 1946. The natural vegetation of Trinidad. Ox- Pholcidae araneae in the French Antilles. Bull. Soc. ford: Clarendon Press. Sci. Nat. 65:3-7. Harris, H. M. 1928. A monographic study of the he- Myers, J. G. 1925. Biological notes on Arachnocoris al- mipterous family Nabidae as it occurs in North bomaculatus Scott (Hemiptera; Nabidae). J. New America. Entomol. Amer. (N.S.). 9:1-97. York Ent. Soc. 33:136-146. Kerzhner, I. M. 1990. Neotropical Nabidae (Heterop- Sewlal, J. N. 2005. Autecological studies of web- tera), 3: Species of the genus Arachnocoris from building spiders. MPhil thesis. Univ. of the West Costa Rica. J. New York Ent. Soc. 98(2):133-138. Indies, St Augustine, Trinidad and Tobago.