110 



EEPORT 108^ U. S. DEPAETMEXT OF AGEICULTUEE. 



Fig. 228.— Hypopus of a TyroglypJius, from be- 

 low. (Author's illustration.) 



usually much before the tip of the body; it is often but an elongato slit, -vrith a sucking- 

 disk each side. In Glyciphogus the openings are much larger, and the genital some- 

 times occupies the entire area betvreen the coxae. 



The internal anatomy of the TjToglyphidip 

 has been carefully studied by Xalcpa and 

 Michael. The digestive system is of the usual 

 type; the ventriculus is very large, with two 

 short coeca, the colon is globose, the rectum 

 very large and open close to tip of body. 

 Behind the anus is a small opening — the 

 copulative aperture. In Glyciphagus the 

 bursa copulatrix projects externally in a 

 small cone. So in the Tyroglyphidae as in 

 the Analgesidae, copulation is not performed 

 through the ^allva, but by this special open- 

 ing. This opening leads to a receptaculum 

 seminis, which connects by a small duct to 

 the ovaries. The nervous system is chiefly 

 concentrated in the very large "brain " which 

 surroimds the oesophagus. The most power- 

 ful muscles of the mite are those attached to 

 the mandibles, the legs, and the stomach. 

 As a rule there is little difference in size between the sexes, but Michael has de- 

 scribed one form with the male not half as large as the female. In several cases there 

 are well-marked secondary sexual characters, such as the enlargement of the first or third 

 pair of legs in the male. In some species the male 

 has two little suckers on the hind tarsi; in others 

 there is a curved plumose bristle on the basal part 

 of the front legs. The transformations of the Tyro- 

 glyphidse are among the most marvelous of the 

 animal kingdom. All Tyroglyphidse (except Car- 

 poglyphus) appear to lay eggs, which are scattered 

 haphazard over the infested material. The young 

 on hatching have six legs, and, molting, obtain 

 two more. Thenceforward their life-history may 

 take the simple and direct path to the adult con- 

 dition, but often it passe sthrough what is called a 

 "hypopus." This hypopus (fig. 228) is A'er>^ dif- 

 ferent from the creature from which it has devel- 

 oped — the octopod nymph. Its body is hard and 

 chitinous; there is no mouth-orifice and no distinct 

 mouth parts. The legs are short and ill-adapted 

 to walking. On the ventral surface of the body 

 near the tip is an area distinct from the general 

 surface and pro\'ided with several circular marks 

 or sucking disks. By means of these sucking disks 

 the Plypopus attaches itself to an insect or other 

 creature and is transported to some other locality, 

 where it may find a suitable brcedmg place. The 

 Hypopus is thus a stage in the life of Tyroglyplms for the purpose of migration. 

 The Hypopus, on finding a suitable locaUty, molts into an octopod nymph, which will 

 feed and develop into an adult mite. The causes that will induce a nymph to trans- 

 form to a Hypopua are not known. M6gnin supposed that the drj^ncss of the air oi: 



Fig. 229. — Histiostomn amcricana. 

 • thor's illustration.) 



(Au- 



