OLIGONEPHRIDIA 271 



Rudiments of mouth parts and legs which are apparent on the second 

 day are not fully developed until the ninth day, although segments of the 

 legs are indicated on the fourth day, and the abdomen is then fully 

 segmented. Muir and Kershaw maintain that the mandibles and max- 

 illae arise as in other insects, the former being articulated in an approxi- 

 mately normal position. The maxillary seta does not represent the 

 palpus but may be a development of the palpiger or the combined 

 lacinia and galea. The maxillary plate represents the cardo and stipes. 



HETEROPTERA 



The Milkweed Bug (OncopeUus fasciatus Dall.) 

 AND THE Fire Bug (Pyrrhocoris a'pterus L.) 



OncopeUus fasciatus, a member of the family Lygaeidae, feeds on the 

 milkweed plant. It is easily reared in the laboratory, since it will feed 

 on the seeds of the host plant and will continue to lay its eggs throughout 

 the year. It therefore is an excellent species for morphological and 

 embryological study. 



The egg is elongate-oval, wdth a pearly white chorion about 1 mm. 

 long. After fertilization, the nucleus divides, and the cleavage nuclei 

 migrate toward the periphery of the egg. About eight hours after the egg 

 has been laid, they may be seen about two-thirds of the way to the surface. 

 Two hours later they are almost within the peripheral cytoplasm, those on 

 the side reaching the periphery before those at the ends, and the nuclei 

 migrating toward the posterior pole reaching their destination last of all. 

 Some nuclei remain in the yolk to form the vitellophags. 



By the fourteenth hour the nuclei are all in the thin cytoplasm at the 

 surface. They are widely spaced in the cytoplasmic layer which is very 

 irregular in thickness. As they increase in number by mitotic division, 

 this irregularity disappears. During the next few hours cell walls appear, 

 and a thin one-layered epidermis of small flattened cells is formed. By 

 the twentieth hour the blastoderm appears thinner on the dorsal side 

 which is the first indication of the differentiation of the serosa from that 

 part of the blastoderm which will form the germ band. The formation 

 of the germ band "corresponds in most respects to the development of the 

 fire bug (Pyrrhocoris) as worked out by Seidel in 1924. On the second 

 day after egg deposition in Pyrrhocoris the germ band, together with the 

 anlage of the amnion, develops in the form of two longitudinal lateral, 

 anteriorly diverging plates made up of cubical cells, the remaining blasto- 

 derm being thinner and destined to form the serosa. This ventral germ 

 band extends cephalad over the anterior pole and caudad to the posterior 

 pole where the two lateral strips converge and fuse. At the posterior pole 

 an invagination of the germ band takes place, the tail end of the develop- 

 ing embryo backing into the yolk to form, together with the amnion, a 



