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CHAPTER 3
LIFE CYCLE

3.1 MATINS

The mating of tsetse flies probably takes place near to or on host animals. Male flies settle on the back of the female, and the claspers at the posterior end of the male abdomen grip the end of the female abdomen. This position may be held for an hour or two, before the male and female flies part company.

Females are mated young, before or at about the time of taking the first blood meal. Females usually mate only once in their lives but some may mate more than once; males can mate several times. Older males are better able to mate successfully than very young ones.

During mating, the male penis is inserted into the vulva, reaching into the uterus as far as the exit of the spermathecal ducts. A large ball of sperm is deposited there in a spermatophore. At the end of mating, the male releases his grip on the female and flies away.

In the next few hours sperm make their way up from the spermatophore into the spermathecal ducts and the spermathecae. Sperm remain active in the spermathecae for the rest of the female's life.

3.2 EGG STAGE

The egg is fertilized immediately it enters the uterus by sperm from the spermathecae coming into contact with and penetrating the anterior part of the egg. The fertilized egg remains lying in the uterus for about four days, while development of the first instar larva takes place inside. The egg is about 1.6 mm long (Glossina morsitans).

3.3 LARVAL STAGES

As with other flies, the larva in Glossinapasses through several stages or instars, as it grows. There are three larval instars in Glossina up to the time when the fully grown larva is dropped by the female fly: the first, second and third instars. The larva has a mouth at the anterior end, and two posterior spiracles. The unusual feature of the Glossina life history is that the larva spends practically all its time, and does all its feeding, within the body of the female fly (Figure 3.1 A).

3.3.1 First instar larva This is the stage that emerges from the egg. It breaks out of the chorion (see 2.5.2.2) using a sharp egg tooth.

The first instar grows to 1.8 mm (G.morsitans)before changing to the next stage by getting rid of its old skin. The first instar lasts for about 1 day.

3.3.2 Second instar larva This is a stage of rapid growth and development. To either side of the posterior spiracles are swellings, and between the spiracles is an area of small spines.

The second instar lasts two days, and the larva grows to a length of 4.5 mm (G. morsitans).

3.3.3 Third instar larva (Figure 3.1 B) This is also a stage of rapid growth and development. The fully grown larva has a pair of large black swellings at the posterior end. These are the polypneustic lobes, which carry many small holes through which the larva breathes. The polypneustic lobes are at first white, becoming black later. The rest of the larva is white in colour. Most of the weight and volume of the third instar larva is due to the gut which contains large amounts of unassimilated food. The third instar lasts just over two days and the larva grows to a length of 6–7 mm (G. morsitans).

Fig. 3.1

Fig. 3.1 Side view of larva of Glossina, A, in the uterus; B, mature larva after being dropped by the female.

Fig. 3.2

Fig. 3.2 Pupal stage and emergence; A, pupa; B, young fly emerging from the pupa, with ptilinum inflated in front of head.

3.3.4 Feeding by the larva (Figure 3.1 A) Apart from the food already in the egg, all the food of the three larval instars comes from the milk gland of the mother fly. The milky secretion of this gland is poured out of the duct of the gland, at the head end of the larva. The larva sucks up this secretion and passes it straight to the midgut. Here it is slowly digested and assimilated.

3.3.5 Breathing by the larva For its air supply the larva depends on air entering the vulva of the female and then passing into its posterior spiracles or polypneustic lobes.

3.3.6 Abortion Sometimes a larva fails to reach its full size and is expelled from the uterus before the usual time. This is called an abortion. The aborted larva dies. Abortions can be caused by the mother fly not obtaining sufficient food, and may also occur when the fly is carelessly handled, or when it comes into contact with insecticide. The egg may also be aborted for the same reasons.

3.4 LARVIPOSITION (BIRTH OF THE LARVA)

When the larva in the uterus is fully grown, the female Glossina flies around looking for a suitable area in which to drop it. This will usually be a place where there is a patch of loose sandy soil, sheltered by an overhanging rock, branch or twig. The female tsetse settles down either on the ground or on the overhanging object. The larva then works itself backward cut of the vulva of the female, helped by pushing movements of the female's legs, and drops to the ground. The larva burrows into the ground and out of sight. The female flies away.

Within an hour or two the larva becomes barrel-shaped, darkens and may then be called a pupa.

There is no feeding by the larva after it is dropped by the female.

3.5 PUPA (Figure 3.2 A)

The pupa is a dark brown rounded object; at the posterior end are the polypneustic lobes the shape of which helps to distinguish the tsetse pupa from the pupae of other flies. The pupa is slightly shorter than the larva that produces it.

The hard case on the outside of the pupa is called the puparium.

Inside the pupa two main processes take place:

  1. the food still remaining in the midgut is digested and assimilated,

  2. the organs of the adult fly begin to form.

The pupal stage usually lasts about four to five weeks, according to temperature. Higher temperatures shorten the pupal period; lower temperatures lengthen the pupal period (to more than 50 days in some climates). Too high or too low a temperature will cause the death of the pupa.

At the end of this period, the adult fly is ready to emerge.

3.6 ADULT FLY

3.6.1 Emergence of the adult fly (Figure 3.2 B) When ready to emerge the young adult fly expands its ptilinum (see 1.3.4) to burst open the end of the puparium. The body works its way out of the hole so made, and also gets through the surrounding soil by using the ptilinum. In this way the young fly struggles to the top of the soil and out into the open air.

At this stage the body is very soft and the wings are small and crumpled. After a few urinates the wings begin to expand to reach their proper size.

3.6.2 Teneral fly (see 7.5) From the time the fly emerges to the taking of its first meal, the young fly is called a teneral fly. The underside of the abdomen appears whitish and semi-transparent, the ptilinum can sometimes be everted when the sides of the head are squeezed between the fingers, and the body has a soft feel to it.

3.6.3 Non-teneral fly (see 7.5) After the first blood meal has been taken, the underside of the abdomen appears more creamy yellow, and when held up to the light the dark shape of the last meal can be seen. The thorax feels firmer and harder, because of the greater development of muscles in it. The ptilinum cannot easily be everted. The fly is then termed a non-teneral fly.

3.7 RATE OF REPRODUCTION

Once mated, a female can produce larvae for the rest of her life.

At a temperature of about 25°C a female fly will produce a mature larva every 9–10 days, except for the first one which may take 18–20 days from the time of emergence of the fly from the puparium (see 8.5.2 and Figure 8.6). Lower temperatures give a lower rate of breeding; higher temperatures increase the rate of breeding. However, temperatures that are too high or too low will cause breeding to stop altogether.


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