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Archive for July, 2010

A recent research by Dr Dmitri Logunov, the Curator of Arthropods of the Manchester Museum, has been devoted to a group of large burrowing wolf spiders (Lycosidae) from Central Asia. A new, unknown to science genus of the wolf spiders, with five new species, has been discovered. Some of the new species exhibit the pronounced differences in sizes between males and females. Males are two or more times smaller than the corresponding females.

 Physical differences existing between males and females of the same species are called Sexual Dimorphism. For instance, differences in body ornamentation could be so elaborate that males and females may even look like different species (Figure 1). Sexual dimorphism also includes body size differences, from moderate to extreme, referred to as Sexual Size Dimorphism. This phenomenon is widespread among spiders. At least seven hypotheses have been proposed to explain the factors that may give rise to size dimorphism in spiders; two of them are mentioned here.

Figure 1. Male and female of the ladybird spider (Eresus cinnaberinus) exhibit a peculiar sexual dimorphism both in colouration and size. ©Vladimir Timokhanov.

Giant females – fertile and attractive

The most spectacular cases of sexual size dimorphism occur in the orb-weaving spiders (Figures 2-3), where dwarf males of some species can be 10 times smaller and 100 times lighter than the females. It is believed that size dimorphism in orb spiders is the result of females becoming giants rather than males becoming dwarfs.

Figure 2. A mating couple of the black-widow spider (Latrodectus dahli) from the Middle East; tiny male is on top. © Barbara Knoflach.

Figure 3. The giant orb-web spider (Nephila fenestrata) from the Gambia, displaying typical sexual size dimorphism: females are huge (some 35 mm long), while males are tiny. © David Penney.

The most common explanation is that large size in females could be driven by the selection on female fecundity, acting to increase the number of offspring produced. Such selection could favour large female size, since larger females can produce more eggs and hence more young. With the high level of juvenile mortality, the production of larger numbers of offspring is crucial for survival of the species. Besides, large females can provide better parental care for their brood. Being bigger also means that females may outgrow their enemies or be themselves more effective predators.

Dwarf males – rushing off to females

Some explanations are based on ecological reasons driving small size of spiders, for instance the differential mortality model. This hypothesis illustrates the evolution of sexual size dimorphism in spiders living at low densities, with large sedentary females and dwarf roaming males (Figures 4-5). It is based on the assumption that due to contrasting life-styles of adult males and females, males suffer higher levels of mortality. This leads to a non-proportionally large number of adult females in the population and, as a result, a reduced intensity of male-male competition. Instead, selection by scramble competition favours males with special traits and/or strategies that enable them to reach females faster. An early maturation of males at a smaller size is advantageous because the quicker males mature, the better their chance of mating. The advantage lies not so much in the small size itself, as in the shorter individual development of males. Males mature in fewer moults than females. This model generally predicts not an absolute selection for the reduction of male size, but for the relative sizes of the two sexes. 

Figure 4. A couple of South African tarantulas (Augacephalus junodi); tiny male is on the left. © Richard Gallon.

For instance, true dwarf males occur in certain tarantulas (Figures 4) and burrowing wolf spiders, living in hazardous habitats characterized by high seasonal aridity and extreme summer temperatures or periodic flooding. In such environments, the burrowing females are safe in their burrows and less at risk than the roving males, which are subject to higher adult mortality. Small males can easier avoid hostile conditions. The reduction of male size could be one of the major adjustments in adapting to such high-mortality habitats.

Figure 5. A tiny male of the crab-spider (Thomisus sp.) from the Gambia sits on the abdomen of the female waiting to mate. © David Penney.

The extreme Sexual Size Dimorphism in spiders is the end result of a complex interplay of various selective pressures. No single hypothesis can fully explain this phenomenon. Each pattern requires its own explanation. Scientists need more life-history and developmental data on dimorphic spider species in order to solve the puzzle of extreme Sexual Size Dimorphism.

Finally, I wish to thank all my colleagues who have kindly provided me with the images used in this post.

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David is a Visiting Scientist in the Faculty of Life Sciences, University of Manchester. His research focuses on spiders and on fossils (including insects) preserved in amber, and also on the invertebrate fauna of tropical West Africa (The Gambia). Regular visits to the Entomology department facilitate the identification of the myriad of creatures he encounters as both fossils and in the living fauna.

David with a sample of copal

The collections of both specimens and literature in the Entomology Department form an invaluable resource available nowhere else in the university. Comparison of his research specimens against this vast wealth of data, permit him to identify most specimens he is interested in, or to determine that they belong to new species, in which case he embarks on their scientific description. For specimens in amber he is currently using the latest imaging techniques, including computed tomography in the Department of Material Sciences.

An image of Amber spider obtained by using the latest imaging techniques, including computed tomography

In addition to his scientific publications, this research has led to the production of several books, including: Spiders in Dominican amber, Field Guide to Wildlife of The Gambia, Field Guide to Butterflies of The Gambia, Common Spiders and Other Arachnids of The Gambia, with several more in preparation (see online for further details).

A collection of scarab beetles of the Manchestert Museum, used by David for writing up on of his books devoted to the bugs of The Gambia

New fossil species in amber discovered recently include two spiders (one in Baltic and one in Dominican amber), one lacebug (Hemiptera) in Dominican amber and one lacewing (Neuroptera) in Baltic amber (see online for further details).

For more information about David Penney’s research visit the Preziosi Lab pages and the amber lab at Siri Scientific Services.

I wish to thank Dr David Penney for providing me with the background of his research programme.

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Of about the million of insect species described in the world, less than 1,500 are known to be a regular part of the human diet. Insects have served as a nutritional, tasty and safe food source for people for tens of thousands of years, all over the planet. The practice of eating insects is known as entomophagy. Insects still remain a popular food in many regions of Central and South America, Africa, Australia and Asia, and entomophagy is a growing industry in over 90 countries. In spite of the fact that bugs can be tasty morsels and provide a welcome course of animal proteins, this diet became superfluous once Man had meat and milk from his own herds. Read more about entomophagy here.

A caterpillar (the witchery grub) of the Carpenter Moth, family Cossidae (left), and the Giant Water Bug (Belostoma sp.) (right) from the insect collections of the Manchester Museum.

The Witchery Grubs, or wood-eating larvae of the cossid moths (family Cossidae) from Australia, are famous food of the Australian Aborigine. They could be eaten cooked or uncooked. The grub grows to about 7cm in size and live up to 60cm below ground. Wood feeding larvae, feeds primarily on the sap from roots of the Witchetty Bush and Small Cooba that is found in central Australia.

 The Giant Water Bugs (Belostoma sp.) is commonly used as food in eastern Asia (China, Japan, Thailand, and others; see also here), where it is used either as food, or as a source of extracts. The spicy sauce made of the Giant Water Bugs is known as namphala (Belostomatid “Essence”) and can be purchased in many Asian grocery stores in the USA.

 Examples of other insect diets can be found online here and here.

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The British fauna used to contain only a few endemic varieties of insects, of which most have already gone extinct. One of them was the British subspecies of the Large Copper (the subspecies dispar) which became extinct from the British Isles in 1865. The extinction of this spectacular butterfly resulted from changing of fenland management and, in particular, the draining of the fens. There have been several introduction attempts, but these have all ultimately failed. The British subspecies, dispar, was endemic to the British Isles and reintroductions have used stock from the continental Europe.

The male (top) and female (bottom) of the Large Copper from the J. Sidebotham collection of British Lepidoptera.

The photographed specimens are from the historically-important collection of the British Lepidoptera by Joseph Sidebotham, acquired by the Manchester Museum in 1919. It is a good example of the Victorian private entomological collections. J. Sidebotham (1824-1885) was a calico printer and JP. His interest ranged from botany and entomology through astronomy and photography. All his specimens are perfectly mounted, reliably identified and are in perfect condition. However, only few of them have locality labels telling us where they were collected from. It is known that the majority of specimens in the Sidebotham Lepidoptera collection were collected in Britain in the late 19th century, but some might have been taken from France as well. Since that time several species of butterflies have already got extinct from the UK, for instance, the Large Copper butterfly represented in this collection by a series of 20 specimens.

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