Introduction 27 



Muscles: Acarinids have striated muscles similar to those of other 

 arthropods. The muscles extend into the appendages from the body 

 and each leg is furnished with extrinsic as well as intrinsic muscles. 

 The body musculature is well developed in many groups. The body 

 muscles are responsible for the movement of the genital armature, the 

 anal plates, and the gnathosoma. Muscles may originate or insert on 

 plates, apodemata, epimera, or on the unspecialized, soft cuticle be- 

 tween the plates. The origins and insertions of muscles can be detected 

 externally because they frequently produce visible scars on the plates 

 and dimple-hke concavities in the softer integument. In the soft-bodied 

 mites the muscles are capable of changing the shape of the idiosoma. 



Digestive System: The digestive system of the Acarina is essentially 

 a simple tube. The anterior portion or fore-gut is derived from the 

 stomadaeum and consists of a muscular pharynx and tubular esopha- 

 gus. The mid-gut or ventriculus is endodermal in origin and is charac- 

 terized by a large lumen and a well-developed digestive epithelium. 

 The hind-gut develops from the proctodaeum and can frequently be 

 divided into an anterior, thin-walled, tubular intestine that in turn 

 empties into a muscular rectum which opens to the outside through 

 the anus. 



There are three fundamental types of digestive systems: 



1. A mesostigmatid type that is characteristic, of the Onychopalpida, 

 Mesostigmata, and Ixodides 



2. A trombidiform type that is characteristic of the Trombidiformes 



3. A sarcoptiform type that is characteristic of the Sarcoptiformes 



The mesostigmatid type has a typical fore-gut composed of a mus- 

 cular pharynx and a long, narrow esophagus that enters a small, central 

 portion of the mid-gut. The mid-gut is characterized by a relatively 

 small ventriculus from which large lateral diverticula arise. The intes- 

 tine is long, except in the ticks, and it opens directly into a spherical 

 rectum which connects with the anus. An ill-defined colon is present 

 in the Notostigmata. Excretory tubules enter the hind-gut between the 

 intestine and colon. 



The trombidiform type is characterized by the lack of colon and 

 rectum. The fore-gut is typical in that the pharynx and esophagus are 

 well developed. The ventriculus is large and its diverticula are broadly 

 attached to it. The hind-gut in the Trombidiformes has been modified 

 to form an excretory organ. Until recently most authors reported that 



