DEVIATIONS IN THE METAMORPHOSIS. 107 



number of statements (not indeed always reliable) made as to post-embryonic 

 development, but also because of the number of variations occurring. "We must 

 therefore refer to the literature already quoted for further particulars, and 

 restrict ourselves to the description of a few ontogenetic peculiarities. 



The formation of the deutovum-membrane in the Acarid egg is apparently 

 very common, and yet it seems to be indisputable that in some forms it does 

 not take place. Claparede, who has given special attention to this point, 

 states that the six-limbed larvae of Tetranychus hatch direct out of the egg-shell, 

 without previously being surrounded by a special chitinous envelope. Tbe 

 appearance of a six-limbed larva also is not universal, although it occurs in 

 most families.* In Phytopta, for example, the larvae are four-limbed, i.e., 

 provided with only two pairs of limbs, and some have been disposed to regard 

 this as a primitive condition. But since, according to Nalepa (Nos. 100 and 

 101), the adult Phytopta also has only four limbs, this must be considered 

 as a secondary condition both in the larva and in the imago. The great 

 preponderance of the abdomen in the Phytopta and the consequent length of 

 the body must also be regarded as a specialised condition. It is interesting, 

 in this connection, to institute a comparison with the Demodicidae, which 

 also have long abdomens. The six-limbed larva is found in its development 

 and, according to Czokor (No. 78), passes through a course essentially agreeing 

 with that described above. 



Taking into account the transformation of the six-limbed larva into the eight- 

 limbed nymph, the occurrence of a four-limbed larva has been thought possibly 

 to denote the more primitive character of the four-limbed form, from which the 

 six-limbed form was to be derived. But we have already shown that such a 

 conclusion is unwarrantable. Some light is thrown on the occurrence of the 

 six-limbed larva by "Winkler's observations of Gamasus crassipes. Although 

 the larva of this form has six limbs, four pairs were distinctly developed in the 

 younger embryo (Fig. 57 A and B). Winkler's account is so clear, that all 

 doubt appears to be excluded. t We must assume that one pair degenerates 

 during a moult that takes place within the egg (formation of the deutovum). 

 Shortly before the embryo hatches, when the limbs are already provided with 

 the characteristic setae, there are only three pairs (Fig. 57 C). This statement, 

 which we are hardly justified in doubting, is a strong argument in favour of 

 the secondary origin of the six-limbed larva. 



The eight-limbed embryos of Gamasus crassipes observed by Winkler appear 

 to be in a lower developmental stage than the six-limbed embryos (Fig. 57 

 A-C). We therefore assume that, in this form, a stage like that found in 

 Pteroptus vcspcrtilionis is left out, this Acarid having an abbreviated course of 

 development. The embryo of Pteroptus commences free life with eight limbs, 

 i.e., at the nymph stage. It could, however, be shown that the embryo 

 passes through a six-limbed stage in the egg while the latter is still within 

 the mother (Nitzsch). X 



Limncsw pardina also leaves the egg as a nymph (Neumann). The young of 

 the Phytopta, when they hatch, are very like the sexually mature adult, having 



* The six-limbed larvae have been observed in the Tetranychidae, Hydrach- 



nidae, Haiaearidae, Oribatidae, Trombidiidae, Gamasidae, Ixodidae, Tyro- 



glyphidae, Dermaleichidae, Sarcoptidae, Demodicidae, etc. 



t [This has since been confirmed by Wagner in Ixodes. — Ed.] 



J [This observation has not been confirmed, and appears very doubtful ; but 



cases probably exist in which the whole hexapod stage is passed through in 



the egg. — Ed.] 



