8 ILLINOIS BIOLOGICAL MONOGRAPHS [8 



up to 1915, and those described since then are listed in the same manner 

 at the end of the present paper. 



The previous monograph covers the species known from the Coleoptera, 

 Orthoptera, and Myriapoda up to the year 1915. The gregarines parasitic 

 in the remainder of the animal kingdom up to the beginning of 1920 are 

 described in the present work. The arrangement of hosts in orders and 

 classes of the Hexapoda follows that given by Comstock (1912). 



Because more and more species are being described in such great genera 

 as Gregarina, Actinocephalus, etc., which are very similar in all their charac- 

 teristics but yet are distinct species, I believe accurate and detailed 

 measurements are necessary for the sporonts, trophozoites, cysts, and 

 spores, or as many of these characters as may be determined. Measure- 

 ments should be made of quiescent but not water-swollen animals while 

 alive on the slide. These measurements must needs be made very quickly 

 after the host is opened for a water medium is highly destructive to the 

 ectosarc of the gregarine, at first causing it to swell out of all normal pro- 

 portions and then to break, with the collapse of the animal. When the 

 parasites do not die within a short time (e.g., the Stenophoridae) they at 

 least lose their original proportions and measurements become valueless. 

 Those taken on preserved material are far from accurate. 



Ratios used in this paper indicate length of protomerite to total length 

 (LP:TL) and width of protomerite to width of deutomerite (WP:WD) as 

 indicated in the author's drawings even though no measurements may 

 have been made. 



One would like to make a clean-cut and sweeping statement that 

 polycystid gregarines are parasites of the arthropods exclusively and 

 unless an exhaustive study be made this may seem to be true. 



But just as higher animals cannot be placed under such definite groups, 

 so it is with these simply organized forms. There is a gradual transition 

 from simple to complex — from the unilocular to the septate forms, and 

 from the simpler to the higher septate organisms. As shown in the group 

 of tables in this paper, I believe there is an interesting and not entirely 

 imaginary parallelism in the character of the hosts which harbor these 

 developing parasites. The transitional forms between non-septate and 

 septate gregarines are found exclusively in the polychaetes. The two 

 lowest (and very similar) families of septate gregarines are found in lowly 

 organized arthropods — the Crustacea and Diplopoda. These gregarines 

 possess no epimerites or very rudimentary ones. 



The next higher family, in which the epimerite is a very simple struc- 

 ture, is abundantly represented in the Orthoptera, but also in the Coleop- 

 tera (which is not one of the lower orders), and is fairly general throughout 

 the Hexapoda. Just as the line of evolution in higher animals may take 

 a single track however aberrent it may prove to be, this is apparently 



