Constitution of the Body in the Blattidse. 229 



The abdominal segments are closed (3) lastly by the " anal 

 piece," into which neither the ventral cord nor the secondary 

 body-cavity is continued, and which remarkably resembles the 

 frontal piece. For on the " anal piece " are also two terminal 

 appendages, originally quite ventral and lobiform, like the 

 antennas, afterwards with tentaculiform terminal appendages, 

 although less developed and later in appearing than the an- 

 tennas, the cerci, which only subsequently move close to or 

 above the anus. 



Further, there is on the anal piece a median dorsal plate 

 above the anus, the anal operculum (lamina supraanalis), 

 and generally two anal valves (valvukc) bounding it laterally, 

 to which an inferior opercular piece is but rarely added. 



The same number of segments as in Blatta occur in all 

 Thysanura, particularly distinctly in Machilis, in which the 

 tenth segment still forms a closed ring, while the strongly 

 developed anal piece is distinguished by three long many- 

 jointed appendages, of which the median one represents the 

 anal operculum and the two lateral ones the cerci. In many 

 of the lower insects and their larvas we also find the same 

 number of segments distinctly marked, as may be best recog- 

 nized in the Acrydia and other Orthoptera, in the larvas of 

 Dragon- flies, &c. ; even in the larva of Hydrophilus R. Heider 

 has demonstrated the occurrence of ten true abdominal seg- 

 ments. 



A comprehension of the variable constitution, especially of 

 the abdomen, of the Hexapoda is possible only from the con- 

 ception of the insect-body founded upon Hatschek's scheme. 

 As will be shown, the divergent conditions can easily be 

 referred back to the primitive condition, such as we have 

 found in the above-mentioned Orthoptera, by citing both the 

 dorsal and the ventral plates of the abdominal segments in 

 simple numbers so far as they are independent and distinctly 

 demonstrable, by furnishing these numbers above with a plus 

 sign ( + ) when the plates are still distinct in the embryo, 

 but in course of development become so aborted and sup- 

 pressed that it usually requires special preparations to render 

 them visible, furnishing them above with a minus sign ( — ) 

 when the plates entirely disappear in the course of the deve- 

 lopment, and entirely omitting the numbers of those segments 

 which are never formed even in the embryo ; and lastly by 

 indicating a secondary amalgamation by a uniting sign (--^) 

 and the anal piece by the letter A, seeing that it is homologous 

 in all forms. As an example how by this schematization an 

 insight into the course of the gradual reduction or amalga- 



