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Archive for the ‘Evolution’ Category

A view of the cockroach collection of the Manchester Museum.

Any online dictionary (e.g., here) can provide a clear definition of what is a human civilization. For instance, it is “the stage of human social development and organization which is considered most advanced”. Such advanced stage is achieved by bringing out of a savage, uneducated or unrefined state, and is commonly measured by a high level of culture, science, industry and government (whatever the latter could mean). Certainly, such definition is rather egocentric and likely to reflect human’s own pride. Possible side effects of any human civilization are rarely considered, not to mention that all such civilizations are developed and thrive at the expense of the Nature surrounding them.

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Fig. 1. A visual history of the cockroaches, from the world it shared with dinosaurs to the urban world it shares with man, by Brian Raszka, 1999 (from M. Copeland, 2003, ‘Cockroach’).

All human civilizations create a specific urban environment, which is not sterile and inhabited by plethora of living beings, such as: rats, fleas, bed-bugs, mosquitoes and other wicked bugs. Collectively they are called synanthropic species, i.e. associated with man. These creatures live with us only because we have provided them with a suitable environment and food. More importantly, their presence is difficult/impossible to control (Fig. 1) – they always are and will be wherever humans do. They share the civilization with us regardless of what we think of them. Thus, why not to accept them as a legitimate part of a ‘human civilization’?

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Fig. 2. Cockroaches as victims of the humans, ‘Executions’ by Catherine Chalmers (from M. Copeland, 2003, ‘Cockroach’).

Cockroaches are among those wicked bugs that are particularly hated by humans (Fig. 2). They are regarded as public health pests, but hardly deserve such a bad reputation. Cockroaches do not sting and do not eat our crops, though may occasionally transmit some pathogens (e.g., salmonella, staphylococcus, etc.) on their feet or their presence may cause an allergic reaction. They have been living alongside the man for hundreds of years, apparently from the time of cave man. The main problem with cockroaches seems to be that we cannot control them. If the environment is suitable (i.e., the right humidity & temperature and the availability of food) – which is usually correct as far as human dwelling concerned – they will always be there. Thus, if it is us who provide cockroaches with a suitable accommodation and lots of food, should we really blame/hate them for staying with us?

In human dwellings, cockroaches hide in cracks/crevices and service ducting. The following short animation was created by Eifion Crane, a BA Animation student of the Manchester School of Art at the Manchester Metropolitan University in 2016. The story tells us about our unwelcomed neighbours who share our civilization with us.

Cockroaches feed on almost anything, from conventional foodstuffs to any kind of organic waste, including faeces. The main reasons why cockroaches become pests are because they are highly mobile, able to feed on almost anything and very prolific. For instance, during its life one female of the German Cockroach can produce 8 egg cases of 40 eggs in each, thus giving birth to some 3,200 youngsters.

There are about 4,500 described cockroach species worldwide (compare with 5,400 described mammal species); of them about a dozen are considered pests. Cockroaches are one of the oldest insects on the planet, dating back 350 million years (Fig. 3). As Don Marquis put it in his ‘Archy and Mehitabel’ (1913), “…I do not see why men should be so proud, insects have the more ancient linage…”.

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Fig. 3. The comparative evolutionary history of the cockroaches and humans, based on Lippman cartoon (from M. Copeland, 2003, ‘Cockroach’).

Cockroaches are gregarious, tending to live in large groups and fouling the environment with their droppings, castings or regurgitated food; they also produce specific smell. This is why in most human cultures cockroaches represent the clichéd symbol of dirtiness, and their presence can cause great distress to housekeepers. The most common house cockroach-mates in Britain (Fig. 4) are the Oriental Cockroach (Blatta orientalis), German Cockroach (Blatella germanica) and American Cockroach (Periplaneta americana).

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Fig. 4. Oriental (two on the left), German (in the centre) and American (right) Cockroaches; from the collection of the Manchester Museum, UK.

Is there any real remedy to get rid of cockroaches? Well, at least one can be suggested straight away. Based on the experience of our ancestors from the 19th century, it could be prudent to appeal to cockroaches’ common sense and intelligence, and to write them a letter: “Oh, Roaches, you have troubled me long enough, go now and trouble my neighbours”. This might help, but if not, then you are right: these cockroaches do not belong to such advanced civilization in which we all live. Something else is to be done (e.g., see here or here).

Cockroaches have had the long-standing relationships with humans, living alongside them since cave dwelling, and will apparently live after we’ve long gone. Knowing that the lethal dose of radiation for a cockroach is many times higher than for a man, one can say with certainty that they are more likely to survive an atomic explosion than us. Cockroaches have a resilience to survive, thriving off our cast-offs, and as humans, we have unknowingly fostered the creatures, which became part of any human civilization.

The following short animation was created by Emily Dobson, a BA Animation student of the Manchester School of Art at the Manchester Metropolitan University in 2016. Music composed by Peter Byrom-Smith and performed by the Guild Hall Collective, conducted by Rod Skipp. Enjoy the animation.

It seems that now there are fewer/no cockroaches in many houses than there used to be. Some say that this is because of electromagnetic waves generated by computers, smartphones and other gadgets we all use. Hooray!  The final remedy to get rid of cockroaches is found. However, is it really a good thing not to have cockroaches in/around our dwellings? If even cockroaches – the most resilient creatures on the planet – cannot survive in our dwellings, we could ask ourselves whether such dwellings are really healthy and suitable for us?

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The fable of the cockroach and the housewife, both do have the long-standing relations (from M. Copeland, 2003, ‘Cockroach’).

Finally, we do need cockroaches to thrive and be around; if they gone, the existence of our own civilization will be at great danger as well.

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Many of the visitors to the Manchester Museum’s Entomology store are researchers, studying various aspects of insect diversity, taxonomy and even physical properties of their colour. A group of researchers from the New Castle University (UK) is interested in what could be a real colour of butterflies and moths seen as if through the eyes of their predators, birds in particular. Here is a brief report provided by Matthew Wheelwright (Fig. 1), a postgraduate student who is involved in this research project:

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Fig. 1. Matthew Wheelwright, a postgraduate student from the New Castle University (on the left) and Phillip Rispin, a curatorial assistant from the Manchester Museum (on the right), are sorting out lepidopteran specimens for scanning.

Colour is vitally important for many aspects of insect lives. It can help them to control their body temperature, or allow them to be recognised by members of their own species. The right body/wing colour can allow insects to blend into their environment in order to hide from predators. Another way in which colour can be used to escape predation is through giving a clear message to their potential predators that they are toxic, not edible or unpleasant tasting and should therefore be avoided. Such insects are usually brightly coloured, with a mixture of yellow/orange and black stripes and spots on their wings. This phenomenon is known as warning coloration (=aposematism). Some other species which occur in the same areas can also benefit from these warning signals by evolving to look like these not edible species; this phenomenon is known as mimicry.

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Fig. 2. Butterflies and moths from the collection of the Manchester Museum sorted out for scanning by means of a hyperspectral camera.

The purpose of our study is two-fold. Firstly we want to find out what makes a good pattern of warning coloration and secondly to discover how closely a mimic must resemble a model having warning coloration (=aposematic model) in order to deceive predators into thinking that they are the same species. In order to do this, we need to know how these patterns look to predators (many of which have different visual systems to humans). We therefore take pictures of specimens from various collections from across the globe, including the entomology collection at the Manchester Museum (Fig. 2), using a hyperspectral camera (Fig. 3). This camera allows us to look at the exact colour spectrum of the specimens, including the amount of Ultraviolet (UV) reflected by them. The latter aspect is very important, as many predators, such as birds, can see in UV.

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Fig. 3 A hyperspectral camera at work, scanning the Black Witch Moth (Ascalapha odorata) from the collection of the Manchester Museum.

We then used models of predator visual systems to quantitatively compare the colour and pattern of aposematic species to non-aposematic species and the patterns of mimics and models to predict how the predators could perceive them and therefore react to them. In other words, we try to see butterflies and moths through the eyes of insect predators and hope to find out whether insects that look aposematic to us (or their mimics) are seen in the same way by their predators.

Two images of the Orange Sulphur (Colias eurytheme) given below (Fig. 4) show how the butterfly appears to humans (on the left) and to a bird (on the right). Orange Sulphur looks iridescent under the UV, and the false colour image on the left contains the purple representing where the UV is the brightest and seen to birds. The image was kindly created for us by Olivier Penacchio of the University of St. Andrews.

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Fig. 4. Two views of Orange Sulphur (Colias eurytheme): left – as seen by a human; right – as could be seen by a bird (contains the purple representing where the UV is the brightest). © Olivier Penacchio

Such research project would be impossible without access to museum specimens from large entomological collections such as that of the Manchester Museum. So we would like to take this opportunity to thank Dr Dmitri Logunov and Phil Rispin for their assistance and generosity with the loan of some specimens.

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The Golden Orb-Weaving spider – Nephila clavipes (Linnaeus, 1767) of the family Nephilidae – is known from USA to Argentina. In Costa Rica, it occurs in lowland and premontane tropical rain forests. Females make large aerial webs in which they usually occupy the centre. Orb-web spiders are effective predators and can easily subdue prey that is significantly larger and heavier than the spider (see on photo).

Two females of Nephila clavipes with prey; Costa Rica.

Two females of Nephila clavipes with prey; Costa Rica.

One of the most peculiar characteristic of this species, as well as of other Nephila species, is an extreme sexual size dimorphism, where dwarf males can be many times smaller and lighter than the females (see on photo). Numerous hypotheses have been proposed to explain the factors that may give rise to such size dimorphism in spiders. Some of them are briefly discussed here.

In the case of Nephila, it is argued that that large size in females could be driven by selection on female fecundity (= the potential reproductive capacity), acting to increase the number of offspring produced. With the high level of juvenile mortality, the production of larger numbers of offspring is crucial for survival of the species. Thus such size dimorphism is almost always due to female gigantism rather than male dwarfism.

As was demonstrated for some African species [e.g., Nephila pilipes (Fabricius 1793)], females continue to grow after reaching maturity. The females mature at varying body sizes and instars and then continue to grow by molting the entire exoskeleton except their copulatory organs (=genitalia). Apparently, this is why in Costa Rica Nephila clavipes is represented by mature females of markedly variable body sizes (although, to date, a post-maturity molting has not been described for this species).

In a short video presented below (courtesy of Alex Villegas, Costa Rica) it is shown how a dwarf male of Nephila clavipes is approaching a giant female in its attempts to mate, alas unsuccessfully this tiem. Indeed, the male is to be careful in order not to be mixed up by the female with a potential prey.

Further reading:

Kuntner, M. & Coddington J.A. 2009. Discovery of the largest orbweaving spider species: the evolution of gigantism in Nephila. – Plos; DOI: 10.1371/journal.pone.0007516

Kuntner M., S. Zhang, M. Gregorič, and D. Li. 2012. Nephila female gigantism attained through post-maturity molting. – Journal of Arachnology 40(3):345-347. DOI: http://dx.doi.org/10.1636/B12-03.1

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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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