Superficially, the tsetse fly resembles most other typical flies but, due to some important anatomical difference it is placed in its own separate family the Glossinidae. All 33 species belong to the single genus Glossina. Fossil tsetse flies have been found in 34 million year-old deposits in Colorado, USA, so we know the family is fairly ancient. In Africa, diseases carried and transmitted by tsetse flies kill an estimated 250,00 - 500,000 people a year, 3 million cattle a year and cost an estimated 1-1.2 billion US$ a year (2010 figures).
Tsetse flies have an interesting and rather unusual life cycle. A single female breeds about four times a year and can produce about thirty broods in a life time. However, unlike most other insects, which produce many eggs at a time and so rely on just a small percentage reaching adulthood to maintain the population (and so, incidentally, producing a constant supply of lava as prey for species higher up the food chain) tsetse flies have evolved a different strategy. They produce just a single egg at a time.
Normally, with most higher insects, the eggs are laid and hatch and the lava grow, shedding their skins at stages, so having several 'instars', each instar being larger and sometimes distinctly marked. Female tsetse flies however retain the single egg in a uterus where it hatches. The lava lives and feeds for the first two instars inside the uterus where it is fed on a milk-like substance (I'm not making this up!) produced by a special gland in the uterus, complete with teat (I'm really not making this up!). The tsetse fly suckles its young internally and gives birth to a large offspring in the form of a third instar maggot. This process of egg retention, development and feeding from a gland in a uterus in insects is known as adenotrophic viviparity.
Third instar lava of other higher insects tend to spend a while in this stage laying down their final stores of nutrients to see them through pupation and into adulthood, and sometime even through adulthood since some fully formed insects don't feed at all but simply exist to mate and lay eggs, then they die. The tsetse fly third instar stage lasts only for a short period however, during which it finds some earth soft enough to burrow into, gets itself a few inches below the surface and sheds its skin for the last time to form a hard-cased pupa. This third instar stage lasts such a short time that it has rarely been observed in the wild. We know about it from observing laboratory strains.
What this process means is that the tsetse female has to supply her single offspring with enough nutrient to grow an entire adult tsetse fly because the lava doesn't feed at all outside her body but pupates and develops into an adult fly using only the nutrients she supplies to it. This represents a huge investment on the part of the female who not only has to find a mate but also enough food for itself and its developing offspring. It literally needs to eat for two.
It does this by feeding off the blood of mammals, which it finds with a sophisticated heat-seeking system.
|Trypanosomes in blood film (stained)|
Tsetse flies don't appear to be affected by the trypanosomes although it is possible their behaviour could be modified. Humans, however, suffer from the debilitating and fatal disease, trypanosomiasis or sleeping sickness. Their livestock suffer from a variety of diseases including nagana from the Zulu N'gana meaning powerless/useless. Most wild African animals are resistant to it, having evolved for millions of years in its presence. Domestic animals, mostly imported into Africa in the last few thousand years however have no such resistance.
This begs the question why then are humans not resistant since they and their ancestors have lived in Africa for many tens of millions of years. The answer to this is that it is very likely, because of our narrow genome range, that humans went through an evolutionary bottleneck of maybe just a few thousand individuals possibly just an isolated group in one small area. We also know that some of our closest ancestors lived in the Afar region of Ethiopia, from which the tsetse fly in absent, so we may all be descended from a population which never actually had much contact with the tsetse fly until relatively recently. Unlike zebra, wildebeest, elephant, giraffe and rhinos, we are comparative newcomers to much of Africa.
The tsetse fly, together with it protozoan parasite, probably had a profound impact on human development in sub-Saharan Africa where it prevented two things which were of major importance to the rest of Euro-Asia and North Africa:
- Establishing herds of cattle to supply meat and (later) milk and dairy produce like butter and cheese.
- Using the horse as a beast of burden and as a source of energy for work such as ploughing, harvesting and threshing, so manpower remained the sole source of energy. A man can just about, using only his own labour, feed himself and his family by agriculture, with maybe a small surplus for trade or barter if the soils is especially fertile and watered. Otherwise the only existence possible without a source of energy greater than manpower is that of hunter-gatherer. Anything more requires at least beasts of burden and draught like horses, donkeys or oxen
What has all this got to do with Creationism and 'Intelligent Design'? Just a couple more things to point out, then I'll get on to that and pose a few questions for those Creationists who have managed to get this far. If they've seen the questions coming they'll most likely have scuttled off somewhere by now to save the embarrassment of avoiding them later.
The trypanosomes which tsetse flies infect humans and their livestock with, is a flagellate protozoan, i.e. it has a flagellum. The flagellum is one of 'Intelligent Design' exponent-in-chief, Michael Behe's favourite example of 'irreducible complexity', a concept with which he made his name and his fortune and which he still pushes as scientific proof that the flagellum could not have evolved and so must have been intelligently designed. This is despite the evolution of the flagella being known at the time he wrote his book (Darwin's Black Box) and despite being forced to admit under cross examination in the Kitzmiller vs Dover Area School District trial that there was no peer-reviewed scientific support for intelligent design.
But okay, for the purpose of this blog, let's let Michael Behe have his intelligently designed tsetse fly trypanosome flagellum in the knowledge that it'll come back to bite him soon.
The other little snippet of information about the tsetse fly is one I personally find fascinating, showing as it undoubtedly does, how genes acting selfishly can produce mutually beneficial cooperation, contradicting Creationists claims and quite paradoxical to what one might expect from a superficial understanding of evolution.
Each tsetse species harbors from one to three prokaryotic symbiont species, and these symbionts may provide opportunities to reduce the vector competence of tsetse flies. The most important symbiont, Wigglesworthia glossinidia, resides in a special bacteriome in the anterior part of the midgut and probably was a symbiont in the ancestor of all extant tsetse species; it likely produces one or more substances that are essential for tsetse reproduction. Sodalis glossinidius, a secondary symbiont not known to be essential for any tsetse species, is found in the midgut and other tissues of several tsetse species. Wolbachia is found in the gonads of some tsetse species and is probably inherited through a strictly maternal lineage. Its effects on tsetse flies have not been established, although in other insects Wolbachia has a variety of effects on their hosts, including inducing cytoplasmic incompatibility.So, tsetse flies would not now exist if it were not for a group of bacteria with which they have formed a symbiotic alliance and which are essential for their reproduction. For that matter, neither would their co-sybiont Wigglesworthia glossinidia. This alliance evolved out of a host-parasite relationship because it was mutually beneficial to both species.
Okay, so lets look at this from the point of view of an intelligent design proponent:
Surely, the tsetse fly is a wonderful example of intelligent design, isn't it? It has beautifully 'designed' heat-seekers system so it can find the highly nutritious food it needs to supply both itself and the young tsetse fly growing inside it. It has wings which enable it to fly like an Exocet straight onto its target once the heat-seekers have locked on. It avoids the waste of laying lots of eggs and producing lots of offspring only as food for other species like so many other insects do, by protecting and nurturing its young in a specially designed uterus completes with milk-producing breast and teat. And, because most African wild mammals are immune to the parasites it carries, it can feed without risking killing its hosts - always a useful strategy for a parasite.
The only problem is that it is quite easy to explain how ALL of these systems could have evolved, and we know that they have had enough time to evolve because we know that tsetse flies have been around for at least 34 million years, so we can explain them all by a known, observable natural process without needing to add the infinite complexity of an unexplained supernatural entity whose own 'design' has to also be explained and which itself remains not even a hypothesis but a mere notion (and merely one notion amongst an array of such notions limited only by human imagination) so multiplying entities needlessly. Hence evolution is the most vicarious hypothesis and the only one which is scientific in that it is the only one which is theoretically falsifiable and the only one to contain verifiable entities.
Evolution also explains the otherwise inexplicable inclusion of an obligate symbiotic bacteria in the tsetse fly reproductive system, including the special anatomical adaptations for housing them. There is simply no rational way this can be described as intelligent design. No amount of special pleading can make this design look like the work of an intelligence in any normally employed use of that term
Why would an intelligent designer infect its creation with trypanosomes, complete with their 'irreducibly complex' flagella? Was it so they would have something nasty to give their hosts by way of a thank you for the meal they just took, or so their hosts would then need to be intelligently designed to resists it? Or was the intelligent designer really only interested in trypanosomes?
And lastly, and this is the hardest point for the intelligent design movement to explain, bearing in mind that they are almost invariably Bible literalists too although they deny any connection and expect us to believe in the fantastic coincidence of them all just happening to be religious fundamentalist, or at least subscribers to the Abrahamic creation myth from the Book of Genesis, including the fundamental belief that the earth and all its creature were created by the 'intelligent designer' for mankind.
|(Trypanosomiasis) Sleeping sickness|
If so, how was this the act of a loving god and how can we distinguish such a god from a malignant, evil god, or a mindlessly unintelligent stupid god whose 'plan' is indistinguishable from no plan at all?
Fortunately, as an evolutionary biologist, I have no such conundrum to cope with so I don't need the mental gymnastics and moral ambivalence Creationists need to cope with these little obstacles reality keeps putting in their way. And I can marvel at the process that created the tsetse fly without needing to dismiss it and without inventing barbaric, patronising, condescending, racist and judgemental reasons to explain what Africans did to deserve it. And nor can I use that latter invention as an excuse not to help do something about it.
Parasites and Vectors - The history of African trypanosomiasis
South African Medical Research Council - Tackling the tsetse fly