Beside these four pairs of appendages, which are in reality modified limbs, there are two median processes, which project one from the top, the other from the bottom, of the mouth, like elongated and hardened upper and lower lips. These are the median labrum above—a deeply grooved structure whose edges approximate and almost touch, thus forming a tube along which the blood of the victim is sucked. Lastly, there is the hypopharynx—sometimes termed the tongue—a median structure a double-edged sword, rising from the bottom of the mouth, and it is this that is the cause of all the trouble.
Fig. 13.—Side view of the head of a female Anopheles maculipennis (magnification about 20), with the various mouth parts separated, but in the relative position in which they lie when enclosed in the groove of the labium. This figure shows the characteristic cephalic scales, a, Antennae; cs, cephalic scales; cl, clypeus; lxe, labrum + epipharynx; mn, mandible; hp, hypopharynx; mx, first maxilla; li, labium; mp, maxillary palps. (From Nuttall and Shipley.)
Fig. 14.—Transverse section through the middle of the proboscis of a female Anopheles maculipennis, showing the relative position of the parts when at rest. Two tracheae and two pairs of extensor and flexor muscles are seen in the labrum. lxe, Labrum + epipharynx; tr, trachea; mus, muscles; hp, hypopharynx; sal, salivary duct; mx, first maxilla; mn, mandible. (From Nuttall and Shipley.)
A glance at Fig. 13 will show how these various mouth appendages can by a skilful use of dissecting needles be separated out, but in nature they are all packed together in a case; the arrangement in the case is shown by Fig. 14, which represents a transverse section of the proboscis. The term ‘proboscis’ is given to the totality of all these structures taken and packed together. With the exception of the labium and of the tactile maxillary palps all the mouth appendages lance into the skin. The proboscis of the male is, however, too weak to pierce the human integument, and it is the female which does all the damage. When a mosquito is going to bite, she alights so gently that her approach is unperceived, and she proceeds to thrust her arsenal of weapons into the epidermis of her victim almost unfelt; the feeling comes later. These weapons are all guided, by the forked end of the softened labium, just as one’s finger-tips guide the end of a billiard-cue. These ‘mouth parts’ are exceedingly fine, extremely sharp-edged structures, whose consistency is about that of whalebone, and both the mandibles and the maxillae have a toothed, serrated edge (Fig. 15). They are partly pushed in by muscles in the head, partly, I think, by the lowering of the body, and they sink slowly and surely into the flesh with as much ease as a paper-knife will penetrate a cream-cheese. But as they sink deeper and deeper into the integument the body of the mosquito approaches nearer and nearer to the skin of the victim, and the labium is pressed farther and farther backwards until at the end of a satisfactory puncture the distal and proximal parts of the labium are parallel and touching.
Fig. 15.—A side view of the labellae and piercing-organs of the proboscis of a female Anopheles maculipennis, dissected out to show the tips of the mandibles, maxillae, and labrum + epipharynx. The hypopharynx is not shown, li, Labium; lxe, labrum + epipharynx; mx, first maxilla; mn, mandible; la, labellae. (From Nuttall and Shipley.)
It is rather an interesting point that the labium does not enter the skin, because the larvae of certain Filarias—one of which produces elephantiasis in man, and the other severe heart trouble in the dog—are found in pairs—probably a male and a female—in the labia of mosquitos. How exactly these nematode larvae leave the labium of the mosquito, and enter the body of the man and the dog, has not definitely, I believe, been cleared up; but that they do enter the human and the canine skin seems certain.
We have mentioned that the labrum is a grooved tube with its edges practically in proximity, and it is up this tube that the blood of the bitten is sucked by the well-known suctorial pharynx which occupies so large a part of the interior of the head of a mosquito. Much the most dangerous weapon of the whole armoury, however, is the hypopharynx. This is shaped like a double-edged sword with a very minute groove running down the centre; this groove is so minute that Professor Nuttall and I and others for some time took it to be a closed tube. It receives at its base the products of the salivary glands of the mosquito, and it is these products which contain the organisms which cause malaria—a disease which has probably caused more trouble and has played a greater part in the history of the world than any other malady to which humanity is heir. Down this minute, microscopic groove has flowed the fluid which has closed the continent of Africa for countless centuries to civilisation, and which has played a dominating part in destroying the civilisations of ancient Greece and of Rome.