Aspects of the Ecology of Four Web Spinning Spiders in West Central Scotland

Abahussain, Mohammed Othman (1990) Aspects of the Ecology of Four Web Spinning Spiders in West Central Scotland. PhD thesis, University of Glasgow.

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The characteristics used to distinguish the four species are described. A comparison was made between sweep netting, vacuum sampling and hand sampling as methods for estimating spider density. Vacuum sampling and hand sampling were of comparable efficiency and were more efficient than sweep netting. Hand sampling was selected as the main sampling method and it was found that 40 minutes was the optimum time for sampling the 2x1 m sample units. Methods far assigning spiders to instar were assessed and it was concluded that the length of tibia 1 was the best feature to use. The use of this feature on field samples gave good agreement with the results obtained from laboratory feeding experiments - where instars can be determined with certainty. The growth rates of the spiders were fairly variable and did not approximate closely to the "growth laws" of either Dyar or Przibram . The energy content of the four species ranged from 20.08-27.39 J/mg. ash free dry weight. Egg sac silk has the lowest energy content and the early instars have lower values than the later instars. Eggs and gravid females had the highest values. Laboratory feeding experiments were carried out on three of the species. In these the spiders developed at similar rates to those in the field. The results allowed energy budgets to be constructed and these indicated that the spiders had high ingestion and growth efficiencies. The life cycles and population age structures of the four species were investigated. M. segmentata and L. triangularis have one year life cycles with five instars. The spiders overwinter in the egg eac and the new generation emerges in the late spring and early summer. The spiders grow very quickly and reach the adult stage in late summer. Egg sacs are produced in the autumn and the females have died by early winter. Spiders in open areas grow faster than those in woodland. L. peltata and some specimens of M. mengei also have a one year life cycle with five instars. The new generation emerges from the egg sacs in mid-summer and grows very quickly. The spiders overwinter mainly as sub---adults and moult to adult the following spring. Egg sacs are produced in the summer and the females die soon afterwards. Some specimens of M. mengei have a two year life cycle with six instars. The new generation emerges in late summer and grows slowly. They overwinter mainly as second instars and grow slowly the following year and reach the adult stage in early autumn. They overwinter for a second time as adults and the females produce egg sacs in early summer and have died by mid-summer. As might be expected, density values are highest as the spiderlings emerge from the egg sacs and decline sharply thereafter. This reflects the high mortality rates. Peak densities for M. segmentata and triangularis were in early summer and for the other two species in the autumn. The average annual densities of the spiders ranged from 1.1/m2 in M. segmentata to 6. 3/m2 in M. mengei . Egg sac production of the four species was investigated. The number of eggs produced ranged from 42 in L. peItata to 96 in M. Mengei . The number and weight of eggs produced was positively correlated with the size of the female. Egg mortality was estimated in the four species. Standing crop values were estimated throughout the year. Peak values occurred in the autumn in all four species and these ranged from 96.6 J/m2 in L. peltata to 260.3 J/m2 in M. mengei . The average annual biomass ranged from 35.2 J/m2 in L. peltata to 107.8 J/m2 in M. mengei. The sex ratio of the four species was investigated in different years using various sampling methods in different vegetation types. The results were fairly variable but, in general, females predominated. The distribution and abundance of the four species was investigated. M. mengei and L. peltata were more abundant in, and more restricted to, woodlands while M. segmentata and L. triangularis were found in woodlands and in open areas. The vegetation structure seemed to be an important factor in the distribution of the spiders and the four species showed some degree of vertical stratification with regard to web height. This may allow the species to coexist in one habitat. The spiders show varying degrees of horizontal and vertical seasonal movement. This may allow them to exploit the resources of the environment more efficiently. The results from thermal death point and light preference experiments were in accord with the distribution of the four species.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Zoology
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78007
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:44
Last Modified: 30 Jan 2020 15:44

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