Cat fleas and chegoes: amazing examples of evolution

Anyone keeping cats is familiar with adult cat fleas (an example is shown below, upper). They may be discovered while combing the fur of our pets, or we may chase individuals we see on bed sheets, sometimes after they have bitten us. The females require a blood meal to gain the nutrients needed to produce eggs and, once laid, these hatch into larvae that live among cat fur, or fall off on to carpets and upholstery in our homes. Although cat owners recognise the adult flea, and its prodigious jumping ability, few know about the larvae (shown below, lower) - or even know of their existence.

Jumping in adults is achieved by extending the long hind legs very rapidly by means of muscles, aided by a pad of a protein, called resilin, at the junction of the leg with the rest of the body. Resilin is one of the most elastic substances known; meaning that almost all the energy stored by distortion is released, with the protein returning almost perfectly to its original form. This is what happens when the leg is extended, contraction of the leg then distorting the resilin pad to again store energy. It is an extraordinary adaptation and, while this special protein is found in a number of insects, where it is associated with flight and the beating of the wings, it has “migrated” through evolutionary time to the hind legs in many types of adult fleas, like cat fleas, that are, of course, flightless. Whatever the explanation for the location of the awesome resilin, it enables fleas to jump impressively on to their hosts - and to avoid potential predators.

While we are annoyed at being bitten, and do all we can to kill fleas, we should remember just what extraordinary insects they are. They have complete metamorphosis, with larvae of quite different form to the adults, and a pupal stage in which the transition of forms occurs. That in itself is remarkable, but so commonplace that we lose our ability to be impressed by its evolution. Add in the structure of the hind legs of fleas, the presence of resilin pads, and their jumping ability and we should be even more amazed.

Chegoes are certainly amazing. Those who travel to tropical Africa, the Caribbean and tropical South America will be familiar with chegoes and readers of Charles Waterton’s descriptive books will have been both fascinated, and horrified, by his accounts of these insects:

The chegoe resembles a flea; and, had you just come out of a dovecot, on seeing it upon your skin, you might easily mistake it for a small pigeon flea; although upon closer inspection, you would surmise that it is not capable of taking those amazingly elastic bounds so notorious in the flea of Europe. [1]

Waterton’s observations on the jumping ability of chegoes is corroborated by other observers and may explain why they attack the lower parts of the limbs of their hosts and why they tend to infest similar regions in humans. I will not provide illustrations of the pathological effects of chegoes (they are not for the squeamish) but leave it to Waterton to provide a description of his contact with the insects during his Third Journey to South America in 1820:

It looks exactly like a very small flea, and a stranger would take it for one. However, in about four and twenty hours, he would have several broad hints that he had made a mistake in his ideas of the animal. It attacks different parts of the body, but chiefly the feet, betwixt the toe nails and the flesh. There it buries itself, and at first causes an itching not unpleasant. In a day or so, after examining the part, you perceive a place about the size of a pea, somewhat discoloured, rather of a blue appearance. Sometimes it happens that the itching is so trivial, you are not aware that the miner is at work. Time, they say, makes great discoveries. The discoloured part turns out to be the nest of the Chegoe, containing hundreds of eggs, which, if allowed to hatch there, the young ones will soon begin to form other nests, and in time cause a spreading ulcer. As soon as you perceive that you have got the Chegoe in your flesh, you must take a needle, or a sharp pointed knife, and take it out. If the nest be formed, great care must be taken not to break it, otherwise some of the eggs remain in the flesh, and then you will be annoyed with more Chegoes.. ..Sometimes I have taken four nests out of my feet in the course of the day. [2]

Unknown to Waterton, chegoes are indeed fleas - “a very sharply defined order of insects without close connexion to any other group” [3] - and they are the smallest fleas yet discovered [4]. 

After landing on a suitable host by walking, or jumping weakly, female chegoes (see below) burrow into the skin and end up with the mouth parts able to suck up blood and the posterior tip of the abdomen only protruding to allow excretion of the dark, blood-rich faeces (a micrograph of a buried female chegoe is shown below - taken from the paper by Eisele et al. [4]). It is thought that mating occurs when the female is embedded, but this is debated currently [5] and an alternative view is that females mate after taking a blood meal from a temporary visit to a host [5]. There is agreement that feeding on blood occurs before mating and, once embedded, a dramatic change in the body of the female takes place. Although he didn’t know it, the “nest” that Waterton describes is the greatly distended abdomen of the female adult flea and it has a volume 2000-3000 times the original and fills with eggs that are then “ejected with considerable velocity” [4]. Most observers find larvae living in sandy soils, where they feed on detritus and it is unlikely that the bloody faeces produced by adults are a significant source of food, as they are, for example, for the larvae of cat fleas. Five weeks after the initial invasion by the female chegoe, the remains of her dead body are expelled from the skin [4] and there is little further adverse effect on the host.

Whatever your reaction to chegoes, it is impossible not to be impressed by their natural history. Humans are very recent in evolutionary terms and the adaptations of these, and other, fleas precede our appearance by millions of years. However, we cannot ignore such organisms when they affect us adversely by their behaviour and we can feel that their attacks are deliberate rather than opportunistic. As a devout Roman Catholic, Charles Waterton’s fascination was certainly anthropocentric and related to his view of the wonders of Creation, although, unlike some other Nineteenth-Century Natural Historians, he did not preach to his audience on the topic. For those who find evolution to be a more pleasing explanation of the extraordinary adaptations of organisms like fleas, there is the perpetual - and intriguing - question of how it all evolved. Was it by a succession of very small changes, or were there sudden, dramatic changes in the genetics of organisms that then controlled their structure and behaviour?

[1] Charles Waterton (1870) Essays on Natural History (edited, with a life of the author by Norman Moore B.A.). London, Frederick Warne and Co.

[2] Charles Waterton (1973) Wanderings in South America, the North-West of the United States, and the Antilles, in the years 1812, 1816, 1820 and 1824 with original instructions for the perfect preservation of birds and for cabinets of natural history (edited with an Introduction by L. Harrison Matthews). London, Oxford University Press.

[3] A.D.Imms (1964) A General Textbook of Entomology (edited by O.W.Richards and R.G.Davies. London, Methuen &Co.Ltd.

[4] M.Eisele, J.Heukelbach, E.Van Marck, H.Mehlhorn, O.Meckes, S.Franck and H.Feldmeier (2003) Investigations on the biology, epidemiology, pathology and control of Tunga penetrans in Brazil: 1. Natural history of tungiasis in man. Parasitology Research 90: 87-99.

[5] M.Thielecke and H.Feldmeier (2013) The fate of the embedded virgin sand flea Tunga penetrans: Hypothesis, self experimentation and photographic sequence. Travel Medicine and Infectious Disease 11: 440-443.

Two bugs, predation and evolution

I grew up by the sea, so it is not surprising that I developed a fascination for marine shores and the plants and animals that live there. ¹ I loved looking in rock pools at low tide and seeing some creatures dart away, while others clung tightly; and lifting drying seaweed to see what sheltered under the wet fronds beneath. As a teenager, there were plenty of other things to occupy my mind, but I retained my love of Natural History and always enjoyed walking through the Devonshire countryside and around the coast. It was while at University that I discovered the flora and fauna of fresh waters and I then went on to have a research career looking at stream and river ecosystems, while retaining a fascination for all aspects of Aquatic Science. It fuelled my teaching and I hope that my curiosity rubbed off on some of my students, as it is enriching. Underlying my interest has always been a child-like approach - “Look at that!”, “Why is that?” etc. - and I’m pleased that I have failed to grow up in that regard.

While collecting damselfly larvae from a weedy lake for a University vacation project, I first came across two creatures that so interested me that I took them home and kept them in large jars. One was a solitary water stick insect, Ranatra linearis (below left, total overall length 7 cm) and then several specimens of the water scorpion, Nepa cinerea (below right, total overall length 3 cm). Although it is similar in appearance to a stick insect, Ranatra is not related to members of this group, but has evolved the same mechanism to avoid detection by resembling a twig. Nepa looks a little like a scorpion, at least at the front end, but it is not related to the scorpions and is very close, in evolutionary terms, to Ranatra, both being members of the Nepidae group of bugs. Nepa is dark brown in colour and resembles the decomposing leaves amongst which it lives so it, too, is camouflaged.

As they are closely-related, one would expect many similarities in the structure, and behaviour, of these insects. Both are true bugs and thus have incomplete metamorphosis, with larval stages that are similar in form to the adults and that undergo a series of moults, and thus increase in size, until the fully-grown insects emerge. During the larval stages, wing buds are present and the adults have wings, although they do not fly well - or often. All aquatic insects have terrestrial ancestors but these bugs spend almost their entire life in water, retaining a system of tubes within the body for air breathing. Whereas some aquatic insects have developed gills containing branched air tubes, both Ranatra and Nepa have two long extensions at the hind end of the body and these are closely applied and form the equivalent of a snorkel that is held at the surface when the bugs need to replenish air supplies.

The majority of insects have six walking legs, allowing them to move over the substratum by having three points of contact at all times, each tripod providing stability. Ranatra and Nepa have six legs, but the first pair is adapted for catching and holding prey, the other four legs being used to crawl slowly through vegetation. Their camouflaged appearance gives the bugs some protection against attack and, together with their lack of movement, makes them good “wait and see” predators. Any suitable prey that comes within range is grabbed by the fore-limbs and, in Nepa, these have become modified so that the lower part folds into a notch in the upper part, much as a blade folds into a penknife (see the video clip below). Any organism that is caught cannot escape and the bug uses its rostrum of piercing mouthparts to both inject salivary secretions and then suck up the body fluids and digested contents of their prey. It was this predation that fascinated me and the bugs that I kept in jars fed on a range of animals, including water fleas and insect larvae of several types. Especially impressive was the capture by Ranatra and Nepa of tadpoles and I was fascinated by their feeding. There’s something about watching predation that captures our attention, perhaps because it seems a little shocking, as we are witnessing "nature in the raw". I saw both bugs complete several meals and was impressed by how little effort appeared to be required from initial capture to the time the remains of the prey were discarded.

                             

Unless one is a Creationist, Ranatra and Nepa provide excellent examples of evolution. They share the same method of catching prey, using modified fore-limbs, feed using a rostrum, they have invaded fresh waters form a previous terrestrial existence, and both have a breathing tube to enable them to remain submerged. It is likely that these were all features of a common, now extinct, ancestor and the two forms then diverged, with the evolution of different body forms that enable them to live in slightly different parts of habitats. Thinking about their structures, and how they evolved, is as fascinating as watching the bugs “in action” and I’m pleased that I’m not a Creationist. Of course, we can all share a sense of wonder in watching Ranatra and Nepa, but to think of evolution, and all the changes that have occurred through time is amazing, as we have no way of conceptualising a million years, let alone the many millions of years over which changes in body form and behaviour occurred. Natural History is not a religion, but it is certainly rewarding to all of us lucky enough to observe all the extraordinary adaptations that evolution has provided.

 

[Please play without sound - you know my views on this. ¹ The red spots on the legs are water mites.]

¹ Roger S. Wotton (2012) Walking with Gosse: Natural History, Creation and Religious Conflicts. Southampton, Clio Publishing.

Meadows, lawns, and human eccentricity

A walk through a lush meadow in spring and early summer is always uplifting, especially when it is sunny and there is a gentle breeze. Each meadow is slightly different in the types, arrangement, and quantities of the wild flowers it contains, with mixtures of scarlet, pink, yellow, mauve, blue, white and other hues, all complementing the background green of the abundant grasses. If left, the grasses and flowering plants produce seeds that disperse and ensure the development of new individuals and some seeds may remain dormant for years, while others germinate in the next growing season.

All meadow plants are fed upon by animals, both invertebrate and vertebrate, and this affects the survival of flowering plants as they may then not be able to produce seeds. Grasses, in contrast, continue to thrive and have evolved to cope with grazing pressure by growing from near the base of the leaves. Grazing encourages the growth of more leaves, and seed heads are produced on rapidly-growing stems, some of which may escape the attention of grazers. Grasses are also perennial, while most flowering plants in meadows are annuals.

At some point in human evolution, our ancestors decided to grow plants for aesthetic reasons, rather than for food production, or for grazing by domesticated animals. We humans are eccentric in developing this practice, when compared to other members of the Animal Kingdom, and our replicates of natural meadows are lawns. Grazing animals are not permitted and have been replaced by repeated mowing to encourage the growth of grasses and to remove other plants. Any flowering plants that appear are now called weeds and we do all we can to control them and the mosses that successfully colonise any bare patches of soil not shaded by the grass blades. The term control is important here as it is a human characteristic that we want to dominate Nature rather than be part of it. Most people do not like meadow plants in their lawns because they prevent the ideal of a uniform growth of grass. As Wordsworth put it:

How does the Meadow-flower its bloom unfold?

Because the lovely little flower is free

Down to its root, and, in that freedom, bold ¹

It is the boldness and freedom of the plants we love to see in meadows that so annoys us when they appear in our lawns.

We like to exert further control by having clear margins to our lawns and many gardeners also prefer the striped effect provided by rolling, first in one direction and then in the opposite one, successively across the lawn. This is considered to be the epitome of a lawn and large expanses of carefully mown grass feature in the grounds of stately homes, their lawns reflecting a larger-scale power over Nature than in most gardens. With a ha-ha marking the beginning of the rural landscape, the landed gentry could truly impress their visitors.

I look after the lawns at my home and keep them as free of moss and meadow plants as I can, with the aid of chemicals and much time spent with a trowel. I certainly have respect for the dispersal capabilities of the plants that appear regularly and wonder at the efficiency of the various mechanisms. By weeding and treating, I am aware that I am arresting a natural process of succession in my lawns and that, if they were not mown, they would become colonised over time by larger, shading plants that would prevent the growth of both flowering meadow plants and grasses, ending up as patches of woodland. Now, I’m not going to allow that to happen.

¹ William Wordsworth - an extract from the 1842 sonnet ‘A Poet!’ - He hath put his heart to school’. The Complete Poetical Works of William Wordsworth (1888). London, Macmillan.