Any continuous or undivided sclerotized plate of the exoskeleton is known as a sclerite and when two sclerites are contiguous the line of junction or demarcation is commonly known as a suture. Sutures in this accepted sense are of three kinds. The most common is a depressed line at the bottom of a shallow furrow or sulcus which marks the position of an internal strengthening ridge. The ridge may be formed in tnro ways.

There have been several speculations as to the origin and significance of the pupa. One that has had considerable vogue is based on the interpretation, by Lubbock (1883), Berlese (1913) and others, of the larva as a free-living embryo prematurely hatched before it has attained the form characteristic of the adult. A nymph, according to this theory, has completed its embryonic development in the egg where it passed through stages corresponding to those of the larva and attained the form of the adult before hatching. This concept assumes, therefore, that the pupal stage is the equivalent of all of the nymphal instars compressed into a single instar. There are many objections to this theory of which the following may be given. Development from the larva to the adult is not continuous. There is evidence that larval and adult structures are determined independently in the embryo, and the development of the one and the suppression of the other are controlled by the endocrine system. Furthermore, changes in hormone balance which take place in the last nymphal instar of the Exopterygota result in the development of the adult, while similar changes in the last larval instar of the Endopterygota bring about the development of the pupa. For this and other reasons the pupa must be regarded as an adult instar and not as a larval or nymphal instar.

The term “protocephalon” as used by Snodgrass (1938) applies to that portion of the head which lies anterior to the mandibular segment. As defined by him it is composed of the “prostomial acron” and a postantennal segment. The term “acron” has been used in somewhat different senses by different authors and to avoid confusion I have introduced the term “blastocephalon” instead. The blastocephalon may be defined as that portion of the head which includes the labrum, clypeus and occulo-antenna1 region. In the germ band it appears as an expanded anterior region, the cephalic lobes, which forms a sort of early embryonic head, hence the name. The posterior trunk-like region of the germ band may then be called the blastocorm.