98 



PS r CHE. 



[June-Sept. 1S94. 



phal character. Leidy ('90) noted very 

 briefly its occurrence in the blue 

 heron.* Kellicott's note ('92) has 

 ah-eadv been referred to ; and to this list 

 must 1)6 added its occurrence in the 

 pitceon at Cambridge, Mass., as found 

 by Woodworth. It will thus be seen to 

 be widely distributed and will no doubt 

 be found in most localities. It may lie 

 hoped that more extended observations 

 will furnish further evidence on its lite 

 history. 



This review should not be terminated 

 without calling attention to a paper by 

 Michael ('S4) which, though it docs 

 not deal with the mite found in the 

 pigeon, is of great interest from its bear- 

 ing on the hypopial stage in other mites. 

 The author first gives a very complete 

 review of the literature on Hypopiis 

 and tabulates at the close of that part of 

 his paper the eight difi'erent views as to 

 what a Hypopiis really is. His own 

 experiments are then given and after 

 showing their entire incompatibility 

 with six of the diverse theories, he says 

 (p. 379) : "My own opinion decidedly 

 confirms Mt'gnin's view that the true 

 Hypopiis is a heteromorphous nvmphal 

 form of Tyroglyphus, and possibly of 

 some allied, or other, genera." At the 

 close of his paper (p. 389) Michael 

 summarizes his conclusions thus: "It 

 appears to me : 



1. That the true Hvpopi are not 

 adult animals, but are a stage in a life 

 history. 



2. That they are heteromorphous 



*There is nn evidence, however, thai tliis is the 

 same species. 



nymphs of Tyroglyphus and some 

 allied genera. 



3. That it is not all individuals that 

 become Hypopi, but only a few. 



4. That the hypopial period takes the 

 place of that between two ecdj'ses in the 

 ordinary life history. 



5. That, in those species which I 

 have examined, the hypopial stage 

 commences with the second nvmphal 

 ecdysis. 



6. That the change to Hypopiis is 

 not caused by unfavorable circum- 

 stances, and is not any extraordinary or 

 exceptional circumstance, but is a pro- 

 vision fif nature for the distribution of 

 the species occurring irrespective of 

 adverse condi'tions. 



7. That, in the present stage of our 

 knowledge, we can no more sa}- why 

 one nymph becomes a Hypopiis and 

 another does not, than we can say why 

 one ovum produces a male and another 

 a female." 



It will be seen from this that while 

 observations and experiments of Michael 

 serve to establish and fix with greater 

 precision M6gnin's view as to the occur- 

 rence of such a stage, the conclusions of 

 the two authors as to the cause of its 

 occurrence are widely at variance. To 

 be sure Michael's work does not touch 

 the hvpopial form found in the pigeon 

 and it is difficult to see how the explana- 

 tion he offers with evident probability 

 for the other forms, i. e. "the distribu- 

 tion of the species irrespective of adverse 

 conditions," could possibly apply to the 

 case of a hvpopial form which like that 

 from the pigeon lives in an inactive 



