8o SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 10 



as a pharyngeal valve, since contraction of the plate muscles would 

 constrict the V-shaped oral aperture. The wing plates of the valve 

 are said by Douglas to be extensions of the pharyngeal wall; the 

 operative muscles, therefore, are evidently the anteriormost fibers 

 of the dorsal constrictors of the pharynx. 



Trombidiformes. — Among the trombidiform mites the chelicerae 

 become progressively adapted for piercing by a transformation of 

 the movable digits into hooks or stylets. In the larvae of the chiggers 

 (Trombiculidae), which are parasitic on vertebrate animals, the cheli- 

 ceral digits are hook-shaped with the points turned upward ; they are 

 used for cutting into the skin of the host, but the mite does not 

 otherwise penetrate the skin. Andre (1927) says the chigger grasps 

 the surface of the host with its palps, and then pushes the cheliceral 

 hooks into the skin. From the puncture of the feeding chigger a tube- 

 like structure extends into the flesh, which was formerly thought to 

 be a sucking organ of the mite, and was named the "stylostome." Its 

 formation, however, as described by Andre, is due to the injection of 

 a digestive liquid by the chigger, which diffuses through the host tissue, 

 producing the wall of the tube and an edematous condition surround- 

 ing the latter, especially at the inner end. Ewing (1944) says the host 

 tissue in immediate contact with the injected fluid "is liquefied, and 

 the adjoining tissue becomes toughened. As the predigested liquefied 

 tissue (not blood) is sucked up by the mite and more digestive fluid 

 is injected into the cavity thus produced there is formed a sclerotized 

 tube which may be as long as the total length of the mite itself." 



The water mites (Hydracnidae), which feed on the larvae of 

 aquatic insects, have long, straight, styletlike cheliceral digits. The 

 feeding and digestion of these mites has been fully described by 

 Bader (1938), who says the mites seize the prey with the palps and 

 tear a hole in the skin with the chelicerae. For from 10 to 20 minutes 

 the mite then quietly holds on to the victim, during which time the 

 congested salivary glands discharge their secretion into the body of 

 the prey and the tissues of the latter are thereby dissolved. Sucking 

 now begins and continues until the mite is replete or the prey is 

 empty of its contents. An Anopheles larva, Bader says, can be sucked 

 dry by three individuals of Hygrobates longipalpus, leaving nothing 

 but the empty skin. After the preliminary digestion by the salivary 

 secretion, the final digestion of the food, as in other Arachnida, ac- 

 cording to Bader takes place intracellularly in the digestive cells of 

 the capacious stomach and its large diverticula. Since these mites 

 have no posterior opening to the alimentary canal, the waste products 

 of digestion accumulate in the stomach cells. 



