Trombidiformes

According to OConnor(1984), the Trombidiformes are characterized by several charcters thatunite the Prostigmata (which makes up the majority of the group) with theSphaerolychidae and Lordalychidae. These characters are: anamorphicsegments AN and PA not added in ontogeny; hysterosomal segment C with fewerthan four pairs of setae; and hysterosomal segments D and E with fewer thantwo pairs of setae. OConnor's cladogram also indicates that the character'hysterosoma without primary segmentation' is a feature of theTrombidiformes; however, as it also occurs in a number of sarcoptiformtaxa, it is not a unique character. Lindquist (1996) notes that most (butnot all) members of the Trombidiformes can be differentiated from theSarcoptiformes by having chelicerae with a hooklike or styletlike movabledigit rather than the ancestral chelate form. Likewise, many trombidiformmites have a padlike or rayed median empodium in contrast to the clawlikeor disk-shaped empodium of sarcoptiforms. Within the Trombidiformes, theProstigmata are united by having the stigmatal openings tothe tracheal systemlocated anteriorly (e.g. on the prodorsum or near the baseof the mouthparts).

View Trombidiformes Tree

from Lindquist 1996

OConnor (1984) performed a cladistic analysis of the major taxawithin the Acariformes and determined that the Sphaerolichida andProstigmata shared two developmental and two setal characters, while theProstigmata were united by the presence of anterior openings to thetracheal system. Lindquist (1996) and Norton et al. (1993) provide recentcladograms of taxa within the Prostigmata. As these relationships arebased on unpublished analyses, it is difficult to discuss the rationale forlinking different taxa; however, Lindquist (1996) does provide a list ofcharacters uniting the superfamily Eriophyoidea.

There are severaldifferences between Lindquist's tree, presented above, and that of Nortonet al. (see below). While both divide the Prostigmata into three majorgroups, the Anystina, Eleutherengona and Eupodina, Lindquist nests theEleutherengona within the Anystina while Norton et al. present them ascompletely separate clades:

                      ================== Parasitengona                   ===|        ==Anystina=|  ================== Anystidae        |          |        |          ===================== other Anystina        |        |             ================== Labidostommatidae        |             |     ===|             |  =============== Eupodoidea     |  |          ===|  |     |  |          |  ===|  ============ Tydeoidea     |  ==Eupodina=|     ===|     |             |        ============ Eriophyoidea     |             |     |             |  ================== Bdelloidea     |             ===|     |                =              ?== Halacaroidea=====|     |                               === Tetranychoidae     |                            ===|     |                         ===|  === Cheyletoidea     |                      ===|  |     |                   ===|  |  ====== Raphignathoidea     |                   |  |  |     |                   |  |  ========= Pterygosomatoidea     |                ===|  |     ==Eleutherengona=|  |  ============ Pomerantzioidea                      |  |                      |  |     ========= Pseudocheylidae                      |  |  ===|                      |  ===|  ========= Heterostigmata                      |     |                      |     ============ Stigmocheylidae                      |                      ================== Paratydeidae

Other differences are that Norton et al. shift Bdelloidea and Halacaroideato the base of the Eupodina, and depict the Parasitengona as relativelyderived within the Anystina. Finally, Norton et al. have moved a number ofgroups typically considered to belong to the Anystina (Pterygosomatoidea,Pomerantzioidea, Pseudocheylidae, Paratydeidae and Stigmocheylidae) to theEleutherengona; however, they do not provide the rationale for thisrestructuring.

Mouth cuts through chitin: velvet mite
 

The mouthparts of a velvet mite can cut through a grasshopper's cuticle due to its knife-like design.

 
  "The red lumps on the feet and legs of this southern lubber grasshopper are not part of its colouring, but are the nymphal stages of the velvet mite. The nymphs hatch from eggs buried in the ground, then attach themselves to a grasshopper, driving their knife-like mouthparts through its cuticle to suck blood." (Foy and Oxford Scientific Films 1982:49)
  Learn more about this functional adaptation.