(4) The hypopharynx, the muscular apparatus of the salivary receptacle, has already been described.
(5) The Labium.-Attached to its basal portion are a pair of spindle shaped muscles; they arise from the under surface of the chitinous prolongations of the maxillae, in the cranium, and are inserted into the groove which separates the labium from the under surface of the head.
(6) The Labellae.-The muscles supplying these mobile appendages at the end of the labium arise from the chitinous projections from the inner surface of the labium. They unite to form a common tendon, and are inserted into a chitinous rod which exists at the base of the labellae. By the contraction of these muscles the labellae are opened and rotated so that their inner surfaces look downward.
These are longitudinal and ventral; the former pass from one segment to the next; they form a dorsal and a ventral lateral group. The ventral muscles pass from the dorsum to the ventral side. By their contraction they flatten the abdomen.
The tracheae, which supply in their finest ramifications the whole of the tissues of the mosquito with air, take their origin in spiracles situated on the surface of the body. There are two thoracic spiracles-one in the mesothoracic and the other in the metathoracic segment. In the abdomen there are spiracles in the pleural membrane of each segment. From the main thoracic spiracle, which is the largest in the body, several branches pass off: (1) A large branch to the head and neck; before entering the neck it gives off a branch to the salivary glands; (2) a branch passing backward and upward supplying the wing muscles; (3) a branch passing backward and downward, from which numerous secondary branches are given off to the thorax; posteriorly it forms a loop with the main anterior branch of the second spiracle. Branches pass off from this loop to supply the fore gut. Each trachea arising from the abdominal spiracles gives off a dorsal and ventral branch, uniting with their fellows on the opposite side and connected with each other by longitudinal commissures. The midgut is supplied by the fourth and fifth while the genital organs are supplied by the sixth and seventh.
The tubes are lined by a single layer of flattened cells limited by a chitinous intima. As the diameter of the tube diminishes, the epithelial cell may embrace almost the whole circumference, the nuclei being curved round. The chitinous layer is thickened in parts, taking the form of a spiral thread. This thread persists until the diameter of the tube reaches about 4 micro. The terminal capillaries lose themselves in the substance of large branched cells which often have a cribriform appearance for this reason. These cells are especially well seen in the ovaries of newly hatched mosquitos.
The nerve ganglia of the mosquito are well developed. In the head there are the large supra esophageal and infra esophageal ganglia surrounding the horizontal portion of the pharynx. These give off nerves to the eyes, antennae, and mouth. From the infra esophageal ganglion pass back two nerve cords, following the course of the salivary duct, to join the large conjoined thoracic ventral ganglion. From this ganglion the limbs are supplied with nerves, and posteriorly it is connected with the chain of abdominal ganglia lying close upon the abdominal sterna. Besides these main ganglia there are also ganglia in connection with the viscera, such as those lying beneath the fore gut and anterior portion of the midgut.
The ovaries and oviduct, the sper mathecae, and accessory glands are comprised in this system in the female. The ovaries are two oval structures lying dorsally to the gut. From these proceed the oviducts, which join below the rectum, forming the common oviduct, which, at first dilated, subsequently narrows. The dilated portion is, according to Kulagin, especially developed at the period of oviposition. It then curves dorsally somewhat forward, and, pursuing a sinuous course, opens beneath the anal opening. At about the point where the oviduct opens into the genital canal open also the ducts (four in Culex, two in Anopheles) of accessory organs, to be described later on. The ovaries in the newly hatched mosquito lie in the fourth and fifth abdominal segments, but they soon enlarge, so as to occupy the greater part of the posterior portion of the abdomen. The size of the ovaries varies from 1/2 to 1 mm. in autumn and winter, to 1 1/2 mm. in spring, when there is an accompanying growth of the eggs. The ovaries are surrounded by a sheath of connective tissue in which the tracheee are embedded. The ovary proper consists of a number of follicular tubes containing the egg follicles. These tubes are radially arranged around a central tube, which is continuous with the oviduct. Each tube is covered with a thin membrana propria, best seen, as pointed out by Kulagin, in fresh specimens teased out in salt solution, and this membrane is, in fact, a prolongation inward of the general peritoneal covering of the ovary. The egg follicles which fill the follicular tubes vary in structure according to their state of development. In a follicle of medium development we find externally the cubical follicular epithelium, while in the interior we have cells of two kinds: at the proximal end, cells which serve to nourish the egg,-the "nurse" cells,-while at the end of the follicle nearest the tube are the egg cells. The number of these nurse cells is three or four. As development proceeds they are absorbed into the ovum, and when the egg is ripe, they have completely disappeared. The follicular epithelium becomes flattened, and forms eventually the chorion of the egg (Christophers). The outer portion of the layer (exochorion) is furnished with oblique parallel markings. The micropylar apparatus is situated at the proximal end, and consists of a globular mass ornamented with rows of pits (Christophers). The oviduct consists of an outer layer in which nuclei are embedded, and a number of muscular fibers; beneath this lies a single layer of flat cells. The portion of this duct beyond the junction of the two oviducts and beyond the dilated portion is the vagina. The epithelial lining is here more highly developed and is cubical in character. Further, the vaginal tube is at this point provided with a thick chitinous coat, an extension inward of the external chitinous integument. Over the opening of the vagina project two flap like processes which are used in the deposition of ova. The accessory glands and ducts in connection with the vagina are four in number (two in Anopheles). Three of these structures are spheric in shape and have a brownish black, chitinous appearance, while the fourth is oblong or clavi form in appearance and is glandular in structure. The spheric bodies are the receptacula seminis or spermathecce, for in the fertilized female the contents of these are found to be spermatozoa, while the function of the gland is supposed to be to supply the cement substance, which, in the case of Culex, serves to bind the eggs together. The gland itself is composed of cylindric epithelium with the nuclei of the cells almost abutting on the lumen. As we have already pointed out, the egg cells may be almost entirely destroyed by an invasion of Sporozoa.