2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I07 



culature of the cockroach bears little resemblance to that of other in- 

 sects. Because of these differences, and the limitation of time for the 

 work, no comparisons have been included ; the vv^ork consists chiefly 

 of anatomical description, with an attempt at explanation of the mech- 

 anism of motion. 



The author acknowledges his deep appreciation to Robert E. Snod- 

 grass for his guidance and counsel throughout the work and for cor- 

 recting the manuscript and proofs. The author further acknowledges 

 his gratitude to the Smithsonian Institution for the facilities to do 

 the work ; and to the Institute of International Education, the Depart- 

 ment of State of the United States, the University of Maryland, and 

 the Ministerio de Ganaderia y Agricultura de la Republica Oriental 

 del Uruguay, which institutions granted the author's fellowship for 

 graduate work at the University of Maryland, of which work this 

 paper is a partial result. Also to Dr. E. N. Cory and Dr. E. E. Havi- 

 land, of the Department of Entomology of the University of Mary- 

 land, the author is indebted for their direction and encouragement 

 throughout his graduate work. 



I. DISCUSSION OF THE MECHANISM OF MOTION 

 THE CERVICAL AND VENTRAL MUSCLES 



It can be observed in a living cockroach, Periplaneta americana 

 (L.), that the head is capable of wide freedom of movement. The in- 

 sect moves it forward or retracts it under the pronotum, or turns it 

 to the right or left side. The head articulates on each side with the 

 apex of the first cervical sclerites (figs. 15, 16, iCv) and on such 

 articulation swings forward and backward. The elasticity of the wide 

 membranous areas of the neck and the folding of the first and second 

 cervical sclerites over its articular line allows the head to make the 

 lateral movements. The muscles responsible for the head movements 

 can be easily seen in the dorso-ventral dissections of the thorax (figs. 

 17 to 20). In a general way it can be said that the muscles inserted on 

 the tentorial bridge are retractors of the head, and the head action is 

 aided by the muscles inserted on the ventral cervical sclerites. Of the 

 muscles inserted on the postoccipital ridge, 52 and 5J are probably 

 retractors too, because they insert approximately at the level of the 

 cephalo-cervical articulations, while 56, 57, and 58, inserted above this 

 level, probably swing the head on the articulations, pulling the upper 

 part of the head backward and consequently bringing forward the 

 lower part (mouth parts). The set of muscles moving the head does 



