Mesostigmata 43 



sternal plates supports this hypothesis. At the other extreme the sternal 

 plate may bear only two pairs of setae. In this case one pair of setae 

 will be found on the jugular plates, while the fourth pair is on the 

 metasternal plates just posterior and lateral to the sternal plate be- 

 tween coxae III and iv. The most common arrangement is to have three 

 pairs of setae and two pairs of pores on the sternal plate and the 

 fourth pair of setae and third pair of pores on the metasternal plates. 

 In the female the genital opening usually lies posterior to the sternal 

 plate and between the metasternal plates. The female genital opening 

 may be guarded by a median epigynial plate that may or may not bear 

 setae; it may be flanked by setal-bearing lateral plates, or it may have 

 both or neither of the plates. A median plate formed by sclerotization 

 of the dorsal wall of the vagina may be present beneath the epigynial 

 plate, or when the epigynial plate has been lost secondarily the median 

 plate may be apparent on the surface as it is in the genus Fedrizzia. 

 Posterior to the female genital opening is the ventral plate. The ven- 

 tral plate bears setae and it may be found fused with either the epi- 

 gynial plate or anal plate. Endopodal plates frequently occur between 

 the coxae and the sternal plates. Parapodal plates occur lateral to the 

 coxae. The stigmata and peritremes are sometimes located on special 

 peritremal plates. Posterior to coxae iv and lateral to the ventral plate, 

 metapodal plates are to be found. In the males the genital opening 

 may be in the middle of the sternal plate or at its anterior border. As 

 a rule much fusion is exhibited in the ventral plates of the males. 



Larvae of mesostigmatid mites have only three pairs of legs and 

 reduced plates. Nymphs have four pairs of legs and can be readily 

 recognized by the absence of genital openings. The dorsal plate of the 

 nymphs is frequently broken into several platelets. The sternal and 

 ventral plates, however, are usually fused to form a single ventral 

 plate. 



Tragardh 1946 has achieved a sufficiently clear understanding of the 

 comparative morphology of the group to be able to explain some of 

 the relationships among the groups. Three of the groups lack an epi- 

 gynial plate and show no evidence of a secondary loss of the plate. 

 These three — the Megisthanina, Liroaspina, and Microgynina — com- 

 prise one group. The other group in which either an epigynial plate 

 or lateral plates or both are associated with the female genital opening 

 is divided into eight groups: the Gamasides, Celaenopsina, Fedriz- 

 ziina, Zerconina, Thinozerconina, Trachytina, Diarthrophallina, and 

 Uropodina. 



