The word anatomy means the structure of the body, in this case, of the tsetse fly.
Like all other insects, the tsetse fly has a tough outer covering or cuticle. The whole of the body is covered with cuticle, even the eyes. Mast parts are hard, but some areas remain flexible, especially the base of the wing, the joints on things and where the mouth parts join on to the head; these parts can therefore be moved easily. The cuticle on the underside (ventral side) of the abdomen in the tsetse fly is elastic, so that it can stretch when the abdomen takes up the large blood meal (see 2.1.6).
Movements of the legs are controlled by muscles attached to the inside of the cuticle of the legs; rapid movement of the wings for flying is controlled by very large muscles in the thorax.
The tsetse flies are nearly always some shade of brown or grey-brown; sometimes there is a slight pink or sandy-red tinge. Several species are very dark. The body usually has darker and lighter patches, making the insect difficult to see when it is settled on bark, rock or soil. At rest, the tsetse normally appears quite slim because the wings are placed one over the other on the back (Figure 1.3), not projecting outwards at an angle to the body as in house flies or most blowflies. Immediately after a blood meal the tsetse abdomen is large, rounded and red.
The body is made up of three main parts: the head, the thorax (to which are attached the wings and legs) and the abdomen. These parts will now be described in greater detail.
Fig. 1.1 Diagram of Glossina, dorsal view, with wings spread out.
Fig. 1.2 Diagram of Glossina, side view, with wings folded.
Fig. 1.3 Glossina, dorsal view, with wings folded.
1.3.1 Compound eyes On the head is a pair of large compound eyes. Each of these eyes is composed of thousands of small units, called ommatidia (singular: ommatidium). The part of the ommatidium that forms the surface of the eye is the lens. The lenses near the midline of the head are slightly larger than those at the sides of the head. The compound eyes of some species are said to be able to detect moving objects at 137 metres (150 yd). They are very good for nearer vision and a small movement near the insect can make it fly off. The compound eyes are dark brown in the living fly.
1.3.2 Simple eyes At the top of the head are three simple eyes or ocelli (singular: ocellus); these are also sensitive to light, but their exact function is un certain.
1.3.3 Antennae (Figure 1.5) There are two antennae placed at the front of the head in a depression between the two compound eyes. Each antenna has three segments of which the third is the largest, and bears the arista.
The arista is a long thin structure like an eyelash, but has a row of branched hairs on its upper side.
The third antennal segment also has on it two small holes leading to the olfactory pits, which contain many sensilla (sensory hairs) that can small the air. The antenna is therefore an organ serving the sense of smell.
1.3.4 Ptilinal suture (Figure 1.4) This is a thin line that partly surrounds the part of the face carrying the antennae. It marks the place where the ptilinum (a balloon-like structure) comes out when the fly begins to emerge from the pupa (see 3.6.1). After emergence the ptilinum is folded back inside the 'head, and only the ptilinal suture can be seen from the outside.
Fig. 1.4 Diagram of head of Glossina, view from the front. Most of one arista has been left out in order to show the ptilinal suture more clearly.
Fig. 1.5 Antenna of Glossina; 1, 2, 3: first, second and third segments of the antenna.
1.3.5 Mouthparts The mouthparts (Figures 1.6, 1.7, 1.8) are very important to the life of the fly. They are long and narrow and can pierce the skin of an animal, so that blood can be sucked up into the fly; at the same time saliva can be passed down the mouthparts into the animal being fed on.
When the fly is not feeding, all the mouthparts are held so that they point forwards from beneath the head.
A pair of maxillary palps help to protect the more delicate proboscis or haustellum which lies between them when it is not in use.
The proboscis is very narrow (Figure 1.6) • It is made up of three parts, the labium, the labrum and the hypopharynx (Figures 1.7, 1.8).
1.3.5.1 Labium The labium is the thickest of these very thin structures. At the free end it has a large number of very small teeth (labellar teeth). The teeth can cut their way through the skin of an animal, so that the fly may suck blood. The other end of the labium, where it is attached to the head of the fly, is swollen. This part (the thecal bulb) contains the muscles that cause the teeth to move.
1.3.5.2 Labrum The labrum forms a tube through which blood is sucked up from the animal being bitten. The tube is called the food canal.
1.3.5.3 Hypopharynx The hypopharynx is an extremely narrow tube through which saliva is pumped into the host animal as the fly feeds.
The thorax in the mature fly is covered by stiff cuticle. The three pairs of legs are attached to the underside of the thorax, and the two wings are attached to the top of the side walls of the thorax.
Fig. 1.6 Side view of head of Glossina;
A, with the mouthparts in the resting position;
B, with the haustellum lowered.
Fig. 1.7 Side view of head of Glossina, with the mouthparts spread out artificially.
Fig. 1.8 Cross section of the haustellum of Glossina.
Fig. 1.9 Leg of Glossina.
Fig. 1.10 Wing venation of Glossina; 1, 2, 3, 4: first, second, third and fourth wing veins.
Just behind the wings, and close to the point where the thorax joins on to the abdomen, are the pair of halteres.
1.4.1 Legs (Figure 1.9) Each of the legs consists of the following segments: coxa, trochanter, femur, tibia and five tarsal segments. The last tarsal segment has two claws and two pulvilli (pads). The coxa is fixed to the thorax and cannot be moved.
1.4.2 Wings (Figure 1.10) At rest, the two wings lie one on the other over the back of the abdomen (Figure 1.3). Each wing has a strong leading (front) edge supported by sane of the wing veins; the trailing (rear) edge is weaker so that the wing membrane may easily be come torn and uneven, especially in older flies (see 8.5.1).
The wing veins form a particular pattern that is characteristic of the tsetse flies:
Vein 1 is very long.
Veins 2 and 3 are very close together throughout their length.
Vein 4 has a strongly curved basal part, before it meets with the anterior cross vein. This causes the cell (area of wing enclosed by veins) immediately after vein 4 to take on a peculiar shape; it is known as the 'hatchet' cell.
1.4.3 Halteres These are two club-shaped organs just behind the base of the wings. They vibrate when the insect is in flight, and help it to steer. They are a sense organ of balance.
1.4.4 Thoracic spiracles There are two pairs of spiracles (or breathing holes) in the sides of the thorax; one pair is just above the point of attachment of the front legs, the other pair is near the base of the halteres.
In the resting fly, the abdomen is covered over by the wings. It has seven visible segments, or parts, and in the male there is, in addition, an extra structure (hypopygium) folded beneath the last two segments (see 1.5.1). On the dorsal (upper) side of the abdomen there are strong plates (each plate is called a tergite), one for each segment; but the ventral side is made of highly elastic cuticle, which can stretch to allow the abdomen to carry the enormous blood meal, and in the case of the female, the large larva. Remains of the blood meal can often be seen within the abdomen if examined from the ventral surface (see 7.6.1) .
There are seven pairs of spiracles (breathing holes) along the sides of the abdomen. The anus is at the posterior end of the abdomen.
1.5.1 The male genitalia (Figures 1.11, 1.12) The word genitalia means parts used for mating. When the male tsetse fly is looked at from the ventral side, a rounded structure can be seen at the posterior end of the abdomen. This is the hypopygium. Just in front of the hypopygium is a plate bearing dark hairs called hectors.
Both these structures make it easy to tell which flies are males (see 8.2). The hypopygium and the hectors help the male to hold on to the end of the female abdomen during mating. At the start of mating, the hypopygium of the male unfolds, uncovering the superior claspers, the inferior claspers and the penis (or aedeagus).
1.5.2 The female genitalia (Figure 1.11) The end of the female abdomen does not have large obvious structures corresponding to the male hypopygium and hectors. But there is a small hole (vulva) through which the larva emerges. This hole may have a few small plates around it; the number and arrangement of these plates can, in some cases, help an expert to identify the species.
Fig. 1.11 Difference between the posterior end of the abdomen in male and female Glossina, ventral view.
Fig. 1.12 Side view of male abdomen of Glossina, to show structures uncovered when the hypopygium opens up.
