©The British Tarantula Society Study Group

HABITAT AND BEHAVIOURAL  DETAILS ON AN IBERIAN THERAPHOSID OF THE GENUS  Ischnocolus

 

Richard C. Gallon

 

Introduction

 

There are believed to be three species belonging to the genus Ischnocolus in Iberia (Smith 1990). This study was carried out on one of these near the southern Spanish town of Alhaurin El Grande (Fig.1). A positive identification was not possible as no specimens were collected. It is most likely that the species in question was I. andalusiacus (Simon, 1873). The spider was essentially black with what appeared to be a pale 'starburst' pattern on the carapace. Information regarding the abdominal colouration was not obtained.

 

The field study was carried out on a cut bank of serpentinite rock at an altitude of 290 metres above sea level between the 24^th and 28^th of July 1994. The bank transect was cut six years previously from undeveloped farm land (Fig.2b). A dirt road was situated above the bank, this separated the study site from natural Mediterranean hill scrub land, believed to be the natural habitat of the species. At the base of the bank a dirt driveway was found which divided the transect from the Macrothele calpeiana site as mentioned in a previous paper (Gallon 1994).

 

The aim of this study was to investigate some of the autecology of this European theraphosid spider.

 

Materials and methods

 

The transect was mapped using tape measure and compass. It was divided up into sectors as shown in Figure 2a. Still photographs and video footage were utilised  to assess the floral ground cover of each sector. Temperature and humidity readings were taken at the weather station thus marked. This comprised of an upturned white garden chair covered with a white bath towel. This set up was intended to shade the instruments from the blazing sun.

 

Specimens were located after night fall by scanning the transect using a halogen head lamp. Efforts were made to search the transect in an unbiased way (i.e. spending a similar duration of time searching each sector).

 

Observations on behaviour were taken while watching the located specimens at night under torch light. Plant identification was carried out during the day and potential predator and prey species were noted throughout the study period.

 

Figure 1: Locality Map

 

 

 

Figure 2a: Sectors of the transect (rotated 90º right)

 

 

Note the numbered pegs on fig. 2a indicate the location of the spider burrows.

The number above the pegs refers to the specimen’s specific number.

The angle of slope is indicated on each line where relevant.

Most sector boundaries have their length, in meters by their sides.

 

Figure 2b: Transect site (rotated 90º right)

 

 

Habitat / Site description

 

The natural habitat was Mediterranean hill scrub. The serpentinite bank transect contained a number of micro-habitats. These can be seen on Figure 2b. They ranged from open, bare, stony soil to shaded rock scree. The vegetal coverage of each area varied from low shrubs to thick grass.

 

In comparing the transect with the surrounding natural habitat the only notable differences between the two  were the existence of the Dwarf Fan Palm (Chamaerops humilis) and the shallower angle of the slope. No doubt the Fan Palm will colonise the transect in the future, thus following in the footsteps of the other vegetation.

 

The stony soil of the area was pale in colour.

 

The total area of the transect was about 340 m². This was split into thirteen sectors who's area, vegetation type and coverage can be seen in Table 1.

 

Graph 1 shows the relationship between temperature and humidity on the transect site. As would be expected temperature rises in the morning, reaches a peak at around midday and then decreases in the afternoon. The temperature range recorded was from 18.5 - 38.5 ⁰c. Humidity decreased in the morning to a low around midday, it then increased in the afternoon. The recorded humidity ranged from 25 - 89 %. Total darkness occurred at 20h27 on the 25^th July 94.

 

Behaviour

 

Ischnocolus was found to be nocturnal, emerging only after sundown. Even then, the five specimens located never emerged fully from their burrow. The front two pairs of legs and palps would be spread out of the burrow entrance, but the carapace and the rest of the spider remained in the burrow as shown in Figure 3.

 

All five burrows were situated in close proximity to large, surface resting stones as seen in Figures 4-7. This seems to be a common feature of the genus Ischnocolus as this has been observed in both I. maroccanus and I. hancocki in Morocco (Smith 1990 & Hancock 1992). Like the latter two species, burrows were sited in areas where there was little or no vegetal cover. In two of the burrows an interesting feature was noted, the presence of a small spherical stone about 10mm in diameter positioned near the burrow entrance. In both instances the stone was firmly embedded in the wall of the burrow. These stones did not seem to have a role in blocking the burrow aperture, but may have some benefit in thermal regulation. I suggest that these stones be called cuniculiths from a combination of the latin for burrow and the Greek for stone.

 

The burrow apertures were mostly ovoid and sometimes silked over lightly during the day. All the burrow mouths were lined with a faint layer of silk which was barely visible. This made burrow identification in day light impractical due to the host of other burrows of non theraphosid origin bringing about confusion. Information regarding the burrows of the five spiders is given in Table 2.

 

Table 1: The herbage cover of the transect

 

 

·The total number of plant species present on the transect was twenty four.

 

·It should be noted that the identification of the flora was based mainly on parched material.  This made positive identification difficult, thus it is likely that some species have been misidentified. 

 

 

 Table 2: Details of the five Ischnocolus burrows

 

 

 

Figure 3: Ischnocolus at burrow

 

 

 

Prey , Competition and Predation

 

The following is Table 3 displaying the species observed on the transect site :-

 

 

 

Key

 

Pred.       = Believed to be a predator of Ischnocolus.

Prey        = Believed to be prey of Ischnocolus.

Comp.     = Believed to compete for prey with Ischnocolus.

 

Discussion

 

From the information gathered at the site it can be concluded that Ischnocolus can recolonise areas of its distribution which have been disturbed by human activity. However the disturbed study site was left free of disturbance for six years after the initial cutting, due to the steep angle of the slope. The dry slope was very unsure under foot, this no doubt was another reason for the lack of human disturbance since the cutting.

 

The species under investigation proved to be nocturnal thereby avoiding the harsh extremes of temperature and humidity. As this spider was dark in colour the sun rays would soon overheat it. The pale colour of the soil would easily highlight this dark spider to any predator. These factors therefore suggest that a nocturnal active period would enhance the survival of the spider.

 

The weather station used in the investigation may have caused slightly discrepant humidity values. Dew was seen to be present on the towel in the morning, this would have caused a micro-climate below, thus leading to higher than normal readings during this time of the day.

 

It has been suggested that Ischnocolus starts a burrow under a large rock as a spiderling and then extends the burrow from this initial scrape (Smith 1990). There is however another advantage other than concealment of young spiderlings, that is, thermal regulation. The adjacent  rock would act as a radiator supplying the spider with a fairly constant temperature as it sits in its burrow mouth. It is possible that cuniculiths also act as a heat source and as they are in close contact with the spider during the night they form a sort of 'hot water bottle'. It is not suggested that the spider moves the stone to the burrow mouth; a more likely explanation is that the cuniculith is already present near the embryonic burrow.

 

Only positively identified burrows were included in the study and it would thus seem likely that there were more specimens present on the transect. Although attempts were made to search the site in an unbiased way, it is possible that specimens may have been concealed by rock scree or dense vegetation.

 

Potential predators and competitors of Ischnocolus are shown in Table 3. As S. melanocephala was seen hunting the slope one morning it would seem likely that poorly concealed Ischnocolus spiderlings would fall prey. This bird would almost certainly take potential Ischnocolus prey. The gecko,  H. turcicus would act in the same way but at night. It is also possible that this gecko may take adult Ischnocolus caught unaware. The unidentified snake would be beneficial as it would eat potential predators like mice.  Many of the invertebrates found on the slope would act as predators and competitors when adult, and form prey when young. No burrow detritus was investigated, so it is not possible to positively confirm that any of the invertebrates formed part of the diet of Ischnocolus.

 

Conclusions

 

ΠThe natural habitat of this spider is believed to be dry Mediterranean hill scrub in southern Spain.

� This species can recolonise areas of human disturbance so long as the site resembles its natural habitat and is free from further disturbance.

Ž The species is nocturnal, emerging after sundown.

� It does not seem to emerge fully from its burrow while hunting.

� The burrow is sited next to a large, surface resting rock with little or no floral cover.

‘ Burrow mouths sometimes have a small stone in them, which I have called a cuniculith.

’ It is possible that the stones in association with the burrow act as heat radiators.

“ Burrow apertures are ovoid or circular in shape.

” Potential predators, competitors and prey have been identified.

 

References

 

Gallon, R. C. 1994.  Observations on Macrothele calpeiana (Walckenaer, 1805) in Southern Iberia. Journal of The British Tarantula Society Study Group, (1): 1-12

 

Hancock, K. & J. 1992.  Tarantulas: Keeping and Breeding Arachnids in Captivity.  R+A Publishing Limited, Taunton, England.

 

Smith, A. 1990.  Baboon Spiders Tarantulas of Africa and The Middle East.  Fitzgerald Publishing, London, England.

 

First edition: December 1994

Second edition: July 1999 (with slight alterations)

The British Tarantula Society Study Group

 

This is the online vertion of :-

Gallon, R. C. 1994.  Habitat and Behavioural details on an Iberian Theraphosid of the genus  Ischnocolus.  Journal of the British Tarantula Society Study Group, (2) (2^nd Edition): 1-14.