External Structure

these regions of sclerotization do not correspond precisely with the primary segmental 59

pattern. The tergal and sternal plates do not cover entirely the posterior part of the primary segment, yet they extend anteriorly slightly beyond the original intersegmental groove. Thus, the body is differentiated into a series of secondary segments (scleromata) separated by membranous areas (conjunctivae) that allow the body to remain flexible. This is termed secondary segmentation. Each secondary segment contains four exoskeletal components, a tergum and a sternum separated by lateral, primarily membranous, pleural areas. Each of the primary components may differentiate into several sclerites to which the general terms tergites and sternites are applied; small sclerites, generally termed pleurites, may also occur in the pleural areas. The primitive intersegmental fold becomes an internal ridge of cuticle, the antecosta, seen externally as a groove, the antecostal sulcus. The narrow strip of cuticle anterior to the sulcus is the acrotergite (when dorsal) or acrosternite (when ventraJ). The posterior part of both the tergum and sternum is primitively a simple cuticular plate, but this undergoes considerable modification in the thoracic region of the body. The pleurites are usually secondary sclerotizations but in fact may represent the basal segment of the appendages. The pleurites may become greatly enlarged and fused with the tergum and sternum in the thoracic segments. In the abdomen the pleurites may fuse with the sternal plates.

The basic segmental structure is frequently obscured as a result of tagmosis. In insects three tagmata are found: the head, the thorax, and the abdomen. In the head almost all signs of the original boundaries of the segments have disappeared, though, for most segments, the appendages remain. In the thorax the three segments can generally be distinguished, although they undergo profound modification associated with locomotion. The anterior abdominal segments are usually little different from the typical secondary segment described above. At the posterior end of the abdomen a varied number of segments may be modified, reduced, or lost, associated with the development of the external genitalia.

Examination of the exoskeleton reveals the presence of a number of lines or grooves whose origin is varied. If the line marks the union of two originally separate sclerites, it is known as a suture. If it indicates an invagination of the exoskeleton to form an internal ridge of cuticle (apodeme), the line is properly termed a sulcus (Snodgrass, 1960). Pits may also be seen on the exoskeleton. These pits mark the sites of internal, tubercular invaginations of the integument (apophyses). Secondary discontinuations of the exocuticular component of the cuticle may occur, for example, the ecdysial line along which the old cuticle splits during molting, and these are generally known as sutures.

Primitively each segment bore a pair of appendages. Traces of these can still be seen on almost all segments for a short time during embryonic development, but on many segments they soon disappear, and typical insects lack abdominal appendages on all except the posterior segments. According to Kukalova-Peck (1987), the ground plan of the insect segmental appendage included 11 podites, some of which carried inner or outer branches (endites and exites, respectively) (Figure 3.21A). All of these podites can be identified in some fossil insects (Kukalova-Peck, 1992), but in the great majority of extant forms only five or six podites at most are obvious, notably in the legs [coxa, trochanter, femur, tibia, tarsus (and pretarsus, which some authors do not consider to be a podite)]. The appendages of the head and abdomen have become so highly modified that homologizing their podites may be extremely difficult. Traces of exites can be seen as gills in some aquatic juvenile insects, and the endites remain as the exsertile vesicles of some apterygotes, but in the majority of insects these branches have completely disappeared.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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