Fig. [239].—Diagram of the viscera of an Oribatid Mite, greatly enlarged. C, C, Lateral caeca of stomach; g, cerebral ganglion; od, od, oviducts; oe, oesophagus; pr.g, pro-ventricular gland; ps, pseudo-stigmatic organ; st, stomach; tr, tr, tracheae. (Partly after Michael.)
Internal Structure.—The minute size of most Mites has rendered research upon their internal structure a matter of great difficulty, and there are still many obscurities to be removed. Those forms which have been subjected to examination present a tolerable uniformity in the structure of the principal organs, but the brief description here given will not, of course, apply to aberrant groups like the Vermiformia. A marked concentration is noticeable throughout the Order, and is best exemplified by the nervous system.
The mouth leads into a sucking pharynx, which narrows to form the oesophagus. This passes through the nerve-mass in the usual Arachnid fashion, and widens to form the ventriculus or stomach. The oesophagus varies considerably in width in the various groups, being very narrow in those Mites which merely suck blood, but wider in vegetable-feeders like the Oribatidae.
The stomach is always provided with caeca, but these are not nearly so numerous as in some other Orders of Arachnida. There are always two large caeca directed backwards, and there may be others. They are most numerous in the Gamasidae (see p. 470), which sometimes possess eight, some being prolonged into the coxae of the legs, as in Spiders. At the sides of the anterior part of the stomach there are usually two glandular bodies, the pro-ventricular glands. In those Mites in which the alimentary canal is most differentiated (e.g. Oribatidae) three parts are distinguishable behind the stomach, a small intestine, a colon, and a rectum, but in most groups the small intestine is practically absent. The Malpighian tubes, very variable in length, enter at the constriction between colon and rectum.
In some of the Trombidiidae there appears to be a doubt as to the existence of a hind-gut at all. A body having the appearance of the hind-gut, and occupying its usual position, is found to contain, not faecal matter, but a white excretory substance, and all efforts to discover any passage into it from the stomach have been unsuccessful. Both Croneberg[[356]] and Henking[[357]] came to the conclusion that the stomach ended blindly, and that the apparent hind-gut was an excretory organ. Michael,[[358]] in his research upon a Water-mite, Thyas petrophilus, met with precisely the same difficulty, and was led to the belief that what was originally hind-gut had become principally or entirely an excretory organ.
The nervous system chiefly consists of a central ganglionic mass, usually transversely oval, and presenting little or no indication of the parts which have coalesced in its formation. Nerves proceed from it in a radiate manner, but no double nerve-cord passes towards the posterior end of the body. As above stated, it is perforated by the oesophagus.
The vascular system is little understood. In 1876 Kramer[[359]] wrote that he was able to perceive an actively pulsating heart in the posterior third of the abdomen in specimens of Gamasus which had recently moulted, and were therefore moderately transparent. No other investigator has been equally fortunate, though many capable workers have sought diligently for any trace of a dorsal vessel in various Acarine groups.
It would appear that the blood-flow in most Mites is lacunar and indefinite, aided incidentally by the movements of the muscles, and perhaps by a certain rhythmic motion of the alimentary canal, which has been observed to be most marked during the more quiescent stages of the life-history.
The internal reproductive organs have the ringed arrangement generally observed in the Arachnida. The two testes, which are sometimes bi-lobed, are connected by a median structure which may serve as a vesicula seminalis, and there are two vasa deferentia which proceed to the intromittent organ, which is sometimes situated quite in the anterior part of the ventral surface, but at others towards its centre. The male Mite is often provided with a pair of suckers towards the posterior end of the abdomen, and sometimes accessory clasping organs are present.