Comparative development of the extensor and flexor tibiae muscles in the legs of the locust, Locusta migratoria

The embryonic development of the extensor and flexor tibiae muscles in the pro- and mesothoracic legs of the locust, Locusta migratoria, is described, and compared to the previously described development of these muscles in the metathoracic legs. The basic pattern of development of each muscle is the same in all three pairs of legs. The extensor tibiae (ETi) forms a giant syncytium, or supramuscle pioneer, which then breaks up into a series of muscle pioneers. The flexor tibiae muscle (FITi) is formed directly by sequential addition of individual muscle pioneers. Thus, there are at least two fundamentally different patterns of muscle development in the embryonic locust, as exemplified by these two muscles, and supramuscle pioneer formation is not a unique feature of the metathoracic ETi associated with its evolutionary hypertrophy. In spite of a basically similar pattern of development of homologous muscles in all three pairs of legs, there are significant developmental differences between the metathoracic ETi and FITi and their homologues in the anterior legs. First, during development of the ETi muscles, the ETi MP forms a double row of attachment sites along both walls of the metathoracic leg, while in the anterior legs there is only a single row. Second, during development of the FITi muscles, the proximal MP, which lies at the tip of the apodeme, dies and breaks down in the metathoracic limbs. In the pro- and mesothoracic limbs it remains intact and eventually forms a large proximal muscle bundle. Third, the accessory ETi and FITi muscles, which develop in the metathoracic legs, are not formed in the anterior legs.