Friday, 23 August 2013

Mixed identity ...


In Man, the destiny of the reproductive glands to develop into either male (testes) or female (ovaries) is determined by the sex chromosomes. The sex of all other parts of the body is controlled indirectly by hormones produced by these glands. These hormones circulate through the body via the bloodstream. The sex chromosomes that are present in all the cells play no part in this process. In insects, however, the situation is very different.

In the early cellular developmental stages of butterflies the sex chromosomes present in every cell decide and control the sex of that particular part of the body. The sex of a butterfly is initially determined by the number of Z chromosomes present in the zygote, the fertilized ovum: if two Z chromosomes are present it will be a male; if only one it will be a female. The zygote subsequently divides and each of these cells will go on to develop into right and left sides of the body. At every cell division there is always a very small possibility of a Z chromosome being lost in the process, causing an imbalance between the autosomes (any chromosome that is not a sex chromosome) and remaining sex chromosome. The resulting cell left with just one Z chromosome changes to being female in character and all further development is along those lines. An individual organism that contains both male and female tissue is termed a gynandromorph. This is the most commonly accepted method by which gynandromorphs arise.

Gynandromorphism in the Lepidoptera is most often associated with wing coloration. Should these features appear combined on the same wing as a mosaic it is referred to as a mixed gynandromorph. But sometimes an insect is bisected, such as the individual below, with one side of the body including the wings being either entirely male or entirely female. These remarkable and rare individuals are termed bilateral gynandromorphs. In many species of butterfly where extremes of sexual dimorphism occur, spectacular gynandromorphs can sometimes arise, particularly where one half is male and the other female.

My thanks to Alec Harmer for his assistance with this article ...

Brimstone, Gonepteryx rhamni (bilateral gynandromorph). Image copyright OUMNH.

Thursday, 15 August 2013

Chasing clouds ...

Clouded Yellow (Colias croceus)

The Clouded Yellow (Colias croceus) is primarily an immigrant to the UK, originating from North Africa and southern Europe, with numbers varying greatly from year to year. Each season, we normally see at least a few reaching our shores and, occasionally, they arrive in large numbers; such as the estimated 36,000 that appeared during 1947 when good numbers of much rarer migrants, such as Bath White (Pontia daplidice) and Pale Clouded Yellow (Colias hyale), were also recorded.

In more recent years, it has been shown that this species has successfully overwintered in the south of England. However, it is believed that the majority of individuals perish, since both larva and pupa of this continuously-brooded species are easily killed by damp and frosty conditions. In good seasons, C. croceus can produce up to three generations in the UK. In flight, the orange-yellow colour is quite distinctive, and unlike any other species. The pale helice form, occurring only in the female, is a creamy white, rather than yellow in colour and may lead to confusion with other similar species.

This strong-flying species always settles with its wings closed and so the dark borders on the uppersides of the wings are only clearly visible when in flight or when viewed backlit at rest. There is no elaborate courtship and, having mated, the female is subsequently able to lay an extraordinary number of eggs; up to 600 have been recorded from a single female.

Two females from 2013 above ...

Monday, 12 August 2013

Precious metals ...

Surrey Downland, 30-31 July 2013

The Silver-spotted Skipper (Hesperia comma) is a warmth-loving species. Its range contracted during the 20th century due to a reduction in grazing stock as well as the onset of myxomatosis in the early 1950s, which severely affected rabbit populations; the butterfly requiring closely-grazed chalk downland sites on which its primary larval foodplant, Sheep's-fescue (Festuca ovina), grows. Recent years have been more promising and this is one of the few species that is increasing its range. It is still relatively-local, with current strongholds being located in the Chilterns, Hampshire, the North Downs between Guildford and Reigate, East Sussex and south-east Kent. It occurs locally in my own county of West Sussex and is currently showing a positive trend for expansion into adjacent suitable habitat.

It is easily identifiable, since it is the only golden skipper found in the British Isles that has the distinctive white spots on the underside of the hindwings, which give the butterfly its name. Like other ‘golden’ skippers, the male is distinguished from the female by the sex brand on its forewings, which is a line of specialised scent scales.

H. comma is one of the latest species to emerge, typically not appearing until late July or early August, and it is then on the wing until early September. There is one generation each year. Like most skippers, this is a fast-flying species that flies close to the ground, and can be extremely difficult to follow when in flight. Both sexes spend the majority of their time either basking or feeding, and a wide variety of nectar sources is used. The butterfly will find the warmest patches of ground on which to bask, enjoying the warmth of paths, rabbit scrapes and other patches of bare earth, which have been baked by the sun. The males rest on suitable sunlit perches, and will investigate any passing butterfly, in the hope of finding a mate. If a virgin female is encountered, the pair exhibits a tumbling courtship, with the male eventually forcing the female to the ground where mating takes place. An egg-laying female locates a suitable patch of bare ground, such as a rabbit scrape, and then walks to the edge of the patch looking for a suitable location on which to lay a single egg.

A female at rest pictured below ...

Sunday, 11 August 2013

Coridon ...

Surrey Downland, 30-31 July 2013

Simply wonderful …

I recently spent two days, with good friend Nick Broomer, searching several areas of chalk grassland on the beautiful Surrey downland; our targets, the beautiful Silver-spotted Skipper and the enchanting Chalkhill Blue. We were not to be disappointed on either count.

Although Tuesday was accompanied by persistent rain for much of the day, an early morning arrival provided ideal conditions for searching the grassland for roosting butterflies; we were not disappointed and found many hundreds, probably thousands, of roosting Chalkhill Blues over much of the hill. This included a number of nice aberrant specimens including those pictured in the two images below, referable to ab. postcaeca, Bright & Leeds (1938). The first image shows a particularly well-marked male which we observed from shortly after emergence until its first flight. Wednesday started warm, and by midday large numbers of insects were on the wing. As the sun pushed through the clouds the numbers of Chalkhill Blues increased and we were privileged to witness a magnificent display as the air and ground around us turned blue as if covered in tiny sapphires. It is difficult to estimate actual numbers, though if I were pressed to do so, there were certainly many thousands and probably many tens of thousands on the wing; possibly many more. Although I didn’t witness the peak of the Sussex Chalkhill Blue explosion last year, Wednesday’s display was an event to remember ...

A privilege to have shared with a good friend …

Wednesday, 7 August 2013

Venom ...

Pirbright Common, 30 July 2013

Vipera berus

The European adder (Vipera berus) is the UK’s only venomous snake. Though painful, adder bites are rarely fatal; with only around 14 recorded cases of death in the last 150 years. As a result of its venomous bite, the adder is a much-maligned species in most of its range. However, they are generally shy, timid and non-aggressive, and bites are most likely to occur when the snake has been disturbed or deliberately antagonised; they should therefore not be intentionally handled. Professional medical help should always be sought as soon as possible after any bite.

V. berus is the most northerly member of the Viper family and is found throughout Britain right up to the north of Scotland. It is not found in Ireland. It shows a preference for open habitats such as heathland, downland, moorland, open woodland and sea cliffs; habitat complexity being essential for different aspects of its behaviour. They rarely stray into gardens.

It is easily recognised by a dark 'zig-zag' stripe along its back. There is also a row of dark spots along each side and a ‘V’ or ‘X’ shape on the back of the head. Its orange-red eyes and vertically slit pupil are also diagnostic. Background colours vary from grey-white in the male, to shades of brown or copper in the female (pictured above). Completely black specimens are occasionally described. They typically grow to around 65cm in length, though can reach up to 90cm, and generally have a rather stocky appearance. Mating takes place during April and May and female adders incubate their eggs internally (ovoviviparous), rather than laying shelled eggs like the grass snake. Adders 'give birth’ to live young in August or September. Their litters ranging in size from 3 to 20 with the young staying with their mothers for a few days. They feed largely on small mammals such as mice, voles and shrews. Lizards, frogs, newts and nestling birds are also taken.

Adders are protected by law in Great Britain. It is illegal to deliberately kill, injure or sell wild adders.