Fig. 388.—Anopheles maculipennis: transverse section through the proboscis of a female (a) and a male (b). hy., hypopharynx, with duct of the salivary gland; m., muscles; md., mandibles; mx., maxillæ; l., labium; l.l., labrum. (After Nuttall and Shipley.)
The abdomen has no limbs, is composed of eight (rarely nine) distinct segments; the sexual and anal orifices are at the posterior end, the stigmata on the sides. The intestinal canal (fig. 389) is composed of three principal divisions; the anterior part reaches as far as the front pair of legs, and consists of the œsophagus, which is provided with two small lateral diverticula. [At the commencement of the œsophagus are one or more diverticula, which vary in size; they contain air, food and bacteria.—F. V. T.] The mid gut reaches as far as the fifth and sixth abdominal ring; in front it is thin, and has numerous small supra-œsophageal ganglia; the posterior part is, however, more dilated. Four or five Malpighian tubes, the excretory organs, discharge at the place where the mid gut passes into the terminal gut.
The pair of salivary glands have one common excretory duct leading into the hypopharynx.
Fig. 389.—Longitudinal section of an Anopheles, showing alimentary canal. In the forepart of the thorax is the salivary gland consisting of three tubules; ventrally, the suctorial stomach extending into the abdominal cavity; the stomach, and at the posterior end of the abdomen the Malpighian vessels. (After Grassi.)
These glandular bodies are situated in the thorax; each consists of three slightly serpentine tubules, the dorsal and ventral tubes being long, the central one shorter. The above-named characteristics apply to both genera Culex and Anopheles, but in the genus Culex is smaller, Anopheles larger. [In Anopheles the ends of the ducts in the lobules are dilated, whilst in most of the genera the ducts are the same size all along. The lobules may bifurcate, and in Psorophora there are five lobules.—F. V. T.] The legs of the genus Culex are about the same length as the whole body; in Anopheles they are double that length.[382] In Anopheles the palpi and proboscis are of equal length; in Culex the condition is different, according to sex. In the male the palpi are longer than the proboscis; in the female considerably shorter and the number of segments diminished. The venation of the wings exhibits further points of differentiation, as also their adornment, though this last sign is not by any means always conclusive; most species of the genus Culex have unspotted wings, whilst those of Anopheles are usually spotted. More important is the fact that in Culex the abdomen is decorated with small scales, similar to those on butterflies, whereas there are small bristles on the abdomen of Anopheles. [This cannot be said to be a character by which an Anopheline may be told from a Culicine, for in such common Anopheline genera as Cellia and Neocellia we get plenty of scales on the abdomen.—F. V. T.] An experienced observer can, however, separate the two genera by the difference in size and their manner of resting. When settled they either touch the resting place with all the legs or only with the four anterior legs. In consequence of the different length of the legs, the body of Culex approaches the resting place more closely; moreover, Culex holds the abdomen parallel or at an acute angle to the resting surface, whereas Anopheles carries the abdomen directed upwards (at an angle of about 145°) and holds the head down. Both genera, however, usually only rest on the four anterior legs, and then, as has long been known, Culex carries the third pair directed towards the dorsum, while those of Anopheles hang down.
In regard to the differentiation of the species, I must refer you to the special literature, and content myself by observing that about 150 species of Culex and about fifty species of Anopheles have been described, of which fifty about four are found in Europe. [The number of known Anophelines now is more—100 species—of other Culicidæ over 700.—F. V. T.] According to our present knowledge it appears that the entire genus Anopheles can transmit malaria to man; this observation has been confirmed in Anopheles claviger, Fabr.; A. maculipennis, Meig.; A. bifurcatus, L.; A. superpictus, Grassi; A. pseudopictus, Gr., all of which are found in Italy,[383] Germany, etc., as well as in the tropics. Moreover, in A. costalis, Loew; A. funestus, Giles (Africa); A. quadrimaculatus, Say (North America), and A. rossii, Giles; the latter is perhaps identical with A. superpictus, Gr., as well as with A. culicifacies (India). [Anopheles maculipennis and A. claviger are the same. Certainly neither maculipennis nor bifurcatus has been found in the tropics. Anopheles quadrimaculatus, Say, is the same as A. maculipennis. There is no evidence that all Anophelines carry malaria, but there is much to show that certain species only are capable of so doing. A list of known carriers is given later.—F. V. T.]