Embryogenesis of the dipluran Lepidocampa weberi Oudemans (Hexapoda,Diplura, Campodeidae): External morphology

The external features of the embryo of the dipluran, Lepidocampa weberi Oudemans are described. The long germ band is formed, and blastokinesis is a simple flexion of the germ band. The primary dorsal organ is formed between the cephalic and abdominal ends by concentration of serosal cells. The mouth fold is formed by ventral extension of the intercalary, mandibular, and maxillary terga, through which entognathy is completed. The posteroventral region of the mouth fold develops into the admentum. Rotation of the labial anlagen is involved in labial formation, and the glossa, paraglossa, and labial palp acquire a tandem arrangement. The postmentum is formed by fusion of the labial subcoxae and is appendicular in origin. The styli and exertile vesicles are derived from the distal parts of bifurcated appendicular anlagen of the second to seventh abdominal segments. The columnar appendage of the first abdominal segment is serially homologous with the exertile vesicles of the following segments. The abdomen is composed of ten segments, and the cercus is the appendage of the tenth, last abdominal segment. Embryogenesis of Lepidocampa weberi resembles that of the rhabduran Campodea staphylinus (Uzel, 1898) as well as that of the dicelluratan Japyx major (Silvestri, '33). It may be emphasized that the rhabduran and dicelluratan diplurans share important features such as entognathy formation and abdominal organization, and the resemblance between them seems to be close enough to postulate their close affinity. Some embryogenetic features, which Diplura and Collembola share, are recognized as plesiomorphic and the manner of entognathy formation may significantly differ. J. Morphol. 237:101–115, 1998. © 1998 Wiley-Liss, Inc.     

Structural mouthpart interaction evolved already in the earliest lineages of insects

In butterflies, bees, flies and true bugs specific mouthparts are in close contact or even fused to enable piercing, sucking or sponging of particular food sources. The common phenomenon behind these mouthpart types is a complex composed of several consecutive mouthparts which structurally interact during food uptake. The single mouthparts are thus only functional in conjunction with other adjacent mouthparts, which is fundamentally different to biting–chewing. It is, however, unclear when structural mouthpart interaction (SMI) evolved since this principle obviously occurred multiple times independently in several extant and extinct winged insect groups. Here, we report a new type of SMI in two of the earliest wingless hexapod lineages—Diplura and Collembola. We found that the mandible and maxilla interact with each other via an articulatory stud at the dorsal side of the maxillary stipes, and they are furthermore supported by structures of the hypopharynx and head capsule. These interactions are crucial stabilizing elements during food uptake. The presence of SMI in these ancestrally wingless insects, and its absence in those crustacean groups probably ancestral to insects, indicates that SMI is a groundplan apomorphy of insects. Our results thus contradict the currently established view of insect mouthpart evolution that biting–chewing mouthparts without any form of SMI are the ancestral configuration. Furthermore, SMIs occur in the earliest insects in a high anatomical variety. SMIs in stemgroup representatives of insects may have triggered efficient exploitation and fast adaptation to new terrestrial food sources much earlier than previously supposed.