74 M. A. Vayssiére on the 
expressing some doubt as to the systematic position assigned 
to it by Latreille. 
It was only in 1869 that my friend and collaborator, Dr. 
Em. Joly, having discovered its habitat in the Garonne, was 
able to ascertain and establish indisputably the true zoological 
affinities of Prosopistoma. 'The presence of trachez in this 
animal showed it to be an insect presenting numerous analogies 
with the larve of the amphibiotic Orthoptera, and especially 
with those of the family Ephemerina. Dr. HE. Joly, nm a 
series of memoirs published either by himself alone or in 
conjunction with his father, Prof. N. Joly of Toulouse, par- 
tially made known the characters of the animal, but could 
never succeed in witnessing its metamorphoses. 
In 1878, having undertaken the whole anatomical part of 
a monograph on the genus, which M. Joly and myself will 
publish shortly, I was struck, during my first dissections, with 
the excessive concentration of the nervous system of this 
animal; and I was thus led to accept the opinion expressed by 
Mr. MacLachlan as to the probably permanent larval condition 
of this insect. Having also zepeatedly observed that the 
genital glands sometimes presented a considerable develop- 
ment, I set myself, from that period, to seek for its reproduc- 
tion in the aquatic state; and it was in pursuing this kind of 
investigation upon some individuals taken in the Rhone at the 
beginning of April this year (1880) that I was able to 
observe, first of all towards the end of the same month, the 
change of skin of these insects (which had not previously been 
noticed), and then on the 3rd of June the metamorphosis into 
the perfect insect of two of them*. 
In this way the question the solution of which I had vainly 
sought during the two preceding years, was at last solved, 
although in a contrary manner to what | had expected. 
These Arthropods therefore, in order to reproduce, followed 
the same course as the other Ephemerina, with which they were 
more intimately connected, not only by this fact, but also 
and especially by the more strongly marked EKphemerine 
facies which they present in the perfect state. 
Before describing this last stage of the Prosopistoma, I 
think it right to say a few words as to the organization of 
the nymph, and to point out the analogies which exist between 
it and the nymphs of the genera Cenzs and Beetisca. 
What especially characterizes our insect in the aquatic state 
is the complete coalescence of the thoracic segments with the 
* T immediately noticed this last fact, in a note which Prof. Blanchard 
was kind enough to present to the Academy of Sciences in the meeting 
of 7th June, 1880. 
