Maxillary and labial palpi are also present, and the latter, together with the labrum or lower lip, form the rostrum.
The bestknown family is the Hydrophilidae, in which the feelers are short with less than eleven segments and the maxillary palpi very long.
In the female of Culex the palpi are much shorter than the proboscis; in Anopheles they are of the same length.
The mouth parts are well developed, consisting of an upper lip, powerful mandibles, maxillae with three-jointed palpi, and a deeply quadrifid labium or lower lip with three-jointed labial palpi.
A somite placed between the prosoma and mesosoma - the prae-genital somite - appears to have belonged originally to the prosomatic series (which with its ocular prosthomere and palpi FIG.
(2) The presence of variously formed scales on the body and its appendages: the head is clothed with scales, the thorax with hairs or scales, and the abdomen with either hairs or scales, or both; the legs and veins of the wings are always covered with scales, and the palpi are often (as in some Anophelinae) conspicuously scaly.
The palpi vary in form and in the number of their component segments, and the proboscis, though usually straight, may be curved (as in Megarhinus) or otherwise modified in shape.
The old genus Anopheles (characterized by the palpi being long in both sexes) is now divided into a number of genera according to the character and shape of the scales on the different regions of the body and on the wings.
In all tsetse-flies the proboscis in the living insect is entirely concealed by the palpi, which are grooved in their inner sides and form a closely fitting sheath for the piercing organ; the base of the proboscis is expanded beneath into a large onion-shaped bulb, which is filled with muscles.
Collectively the Ixodidae and Argasidae may be distinguished from other Acari by the presence of a median probe, armed with recurved teeth, which project forwards beneath the mouth and between the palpi, and of a conspicuous spiracular area above and usually behind the base of the fourth leg on each side.
The mouth parts consist of two small retractile mandibles, of a pair of short palpi and of the toothed probe above mentioned.
In the Argasidae the anterior portion of the dorsal surface of the body is extended forwards above the capitulum, so that this structure is concealed from above; the integument is fairly uniformly granular or coriaceous above and below; the palpi are simple and unmodified; there is no sucker beneath the claws in the adult, and there is only a slight structural difference between the sexes.
In the Ixodidae the capitulum is not overlapped by a forward extension of the dorsal area, which is smooth and firmly chitinized either in front or all over; the palpi are usually modified, that is to say, their second and third segments are usually excavated internally to form a sheath for the hypostome; there is a distinct sucker beneath the claws and the difference between the sexes is well marked, the males having the dorsal integument thickly and continuously chitinized, whereas in the females only its anterior portion bears a chitinous plate, the rest of the integument being soft to admit of its distension by the blood which is imbibed in quantity by members of this sex.
A, Rostrum or hypostome; b, b, Palpi; c, Genital aperture; d, Anal orifice; e, e, Ventral surface of capitulum; g, Sternum; 1 -7, segments of leg.