244 POPULAR SCIENCE MONTHLY 



GEOLOGICAL HISTORY OF COCKROACHES 



BY Dr. E. H. SELLARDS, 

 UNIVERSITY OF FLORIDA 



~^T insects are more abundant as fossils, and none so widely dis- 

 -^ tributed through the various formations, as are the cockroaches. 

 Their delight in moist places, often near the banks of streams, their 

 firmly chitinized wings and body render them of insects among the 

 best adapted for preservation. Especially are they to be expected in 

 association with ferns and other plants among which during life they 

 found shelter, and together with which they were transported to their 

 resting place in the rocks. Hardly ever will persistent search among 

 fossil leaves of land origin, when imbedded in rocks of sufficiently fine 

 texture, fail to bring to light at least detached wings and perhaps 

 bodies. Even when the bodies of adults have not been preserved, not 

 infrequently will be found the cast-off integument of the young. It 

 is this approximately complete geological record that lends an especial 

 interest to the cockroach family. 



The cockroaches have proved themselves a remarkably conservative 

 group having retained throughout their long existence, as compared 

 with other insects, a relatively generalized structure. The develop- 

 ment is direct, the young resembling the adults. The mouth parts are 

 of the biting kind common to primitive insects. The segmentation of 

 the abdomen and thorax is distinct. The foot is five-jointed. The 

 venation of the wing is much less complicated than that of many of 

 the more advanced types. Not all the organs, however, have retained 

 this primitive simplicity. In this, as indeed in every group, some 

 organs have outrun others in degree of specialization, so that the group 

 early became a characteristic and well delimited one. The body is 

 flattened, the head small and turned downwards. The covering of 

 the first thoracic segment, the pronotum, is enlarged, rounded, more 

 or less shield-shaped. The front wing is firmly chitinized and lies 

 fiat on the back, or slightly arched to conform to the shape of the 

 thorax. An inner area near the base of the wing is marked off by a 

 deep curved line, the anal furrow. The hind wing is less resistant and 

 broader, the greater width being obtained by a greater expansion of the 

 inner border. Hardly ever will a doubt arise as to the reference to this 

 family of even a fragment of a specimen. 



The structural characters so far mentioned made their appearance 

 early. The rounded pronotum is as characteristic of Paleozoic as of 



