THE ORIGIN AND EVOLUTION OF THE CLASS INSECTA

SICIEN H.CHEN Institute of Entomology,Academia Sinica

Much has been written on the origin and evolution of the class Insecta.The three classic problems of the subject are: the origin of insects as aclass, the origin of wings and the origin of holometabolism. In the past,these were often dealt with separately, each as an independant problem; noprevious writer has yet been inclined to take sufficient notice of the impor-tance of their interrelationship and mutual counections. As evolutionarysteps are chained, it is much more desirable that the problems, which repre-sent succesive steps in insect evolution, should be considered from the view-point of their chained relations. 1. The Three Principal Steps in Insect Evolution and Their Chained Relations Insects are descendants of Symphyla-like ancestors (Imms, 1947). The onlyfundamental change that is necessary to derive them from their Protosym-phylan ancestors seems to be the concentration of the locomotory function inthe first three postcephalic segments, that is to say, insects could have evolvedthrough the acquisition of a locomotory center. Considering the ancestralcharacters of insects, all evidence seems to indicate that in the ancestral form,the young larva hatched in a very early stage of development, in whichthere were only a small number of postcephalic segments and three pairs oflegs. In other words, the primitive larva possessed a locomtory center at thetime of hatching but it was lost in subsequent development by the acquisitionof additional abdominal segments and legs. The retention and development ofthis larval locomotory center in the adult is the decisive step that leads tothe appearance of the insect class. This is the first and most importantfeature in the history of insect evolution. The second great step in insect evolution is the development of wings.With the acquisition of a locomotory center, the insect body is differentiatedinio three parts-head, thorax and abdomen. This differentiation of bodystructure opens new possibilities for further evolution, and as a consequenceof the development of the locomotory center, wings developed. The possession of wings affords the insects the capacity of flight and ofconquering the air, but it also brings about a definite contradiction betweenthe larva and adult in their conditions of life. For the wings acquire theirfunction only in the adult stage, while the larva of the primitive wingedinsects, as evidenced by those of Epimetabola and Prometabola, probably leftthe egg in a rather early stage of development. With this difference in thebody constitution and the means of locomotion, the larva and adult couldscarcely be confined to the same way of living; and the result of this contra-diction brings about a differentiation in the separate directions of developmentbetween these two stages of insect life. This is the third great step in insectevolution which culminates in the Holometabola. Therefore, it is very clear that these three principal steps form a chainin the history of insect evolution: each has its past conditions as prerequisiteand itself acts as prerequisite for the next. Briefly, we may summarize asfollows: Existence of a locomotory center in the ancestral larva→retention and development of the larval locomotory center in the adult, appearance of insect→further development of the locomotory center, appearance of wings→contradic-tion between the wingless larva and winged adult in conditions of life→diffe-rentiation of the conditions of life and of the directions of development be-tween larva and adult, appearance of Holometabola. 2. The Acquisition of Locomotory Center and the Origin of Insect De Beer (1930) was the first to express the view that insects are neote-nous myriapods. He compared the adult insect with the larvae of Diplopodasuch as Iulus, and suggested that insects have evolved from such a creatureby a process of sexual precocity. or neoteny (fig. 1). His view was met withstrong objections from both Tillyard (1931) and Foxon (1935) on the groundthat firstly, insects are not neotenous diplopods; and secondly, the majorityof diplopod larvae do n