478 MR. A. D. MICHAEL ON THE 



either with boiling water or chloroform; but a large number also have been made on 

 individuals hardened in alcohol and other hardening reagents : this is advantageous for 

 some organs, but usually in Acari I prefer the fresh specimens. The dissections have 

 mostly been stained on the slide with borax-carmine or picro-carmine. The sections 

 have been made from specimens killed with hot water, fixed with picro-sulphuric acid 

 or Flemming's fluid, stained on the slide with ha^matoxylin (Ehrlich's), and mounted in 

 balsam. I may say that Flemming's fluid appears to give somewhat the best results 

 when it penetrates, but that its penetrating power is so inferior to picro-sulphuric acid 

 in the case of Acari that a considerable number of specimens are lost when that reagent 

 is employed. Whereas j)icro-sulphuric acid can be relied on to penetrate such Acari as 

 Bdella with sufficient rapidity it is probably best to use it, although the results will 

 not be quite equal to really good Flemming's fluid-hardened specimens. Other 

 reagents which I have tried have not, I think, produced an equally satisfactory fixing 

 of the histological condition. 



The species which I have used has been chiefly Bdella. Basteri (Johnston, 7 *), 

 because, through the kindness of Professor Herdman and the officers of the Biological 

 Station (at Port Erin, Isle of Man) of the Liverpool Marine Biological Committee, I have 

 received supplies of large numbers of this species at frequent intervals and at various 

 seasons; these have enabled me to pursue the enquiry in a manner which I could not 

 otherwise have done ; and I beg to tender my best thanks for the assistance. I was 

 anxious to obtain this species because it is, in my judgment, the best suited for research 

 of those that I know, being large (for a Bdella) — its total length isa bout 2-5 to 3 mm., 

 and being also very strong and vigorous, with the parts well developed. I have also 

 used Bdella vulgaris, B. capillata (Kramer), and some others, but could not obtain them 

 in equal numbers or with equal certainty. Wherever in the following paper an organ 

 or structure is mentioned without the species of Bdella being named, it belongs to 

 B. Basteri. 



I have adopted the name of B. Basteri for the species because there is no doubt that 

 it is the creature described by Johnston under that title in 1847, while it is quite 

 uncertain whether it is the species referred to by any earlier writer. Thus it has been 

 supposed that B. Basteri is the same species which is called Acariis longicornis by 

 LinnjEus is his ' Fauna Suecica,' but this is very doubtful. I hardly see how the opinion 

 is arrived at. Moreover Andrew Murray f considered that Linngeus has described a 

 different species under the same name in the ' Systema Naturae,' and that this latter is 

 the species to which the name is now generally applied. 



Bdella sanguinea, Trouessart, 1894 (21), wliich that author, following Andrew 

 Murray, places in a subgenus Molgus (Dujardin), is, I think, a synonym ; indeed, 

 Trouessart himself calls attention to the probability of its being so, but was not able 

 to obtain Johnston's original paper. Trouessart does not describe the hairs on the 

 mandibles, which are important according to Kramer's tables of the genus, therefore I 



* This figure, and all similar figures throughout the jjaper, refer to the list of authorities quoted (Bibliography) 

 page 523. 



t 'Economic Entomology: Aptera' (London, 1876), p. 143. 



