THE RENEWAL AND REPLACEMENT OF THE STYLETS OF SUCKING INSECTS DURING EACH STADIUM, AND THE METHOD OF PENETRATION

Abstract

In a study of the mouth parts of the scale and aphis, certain conclusions corresponding to those of Hermann Weber were independently drawn. While several phases of the subject need clarifying, it is definitely established that the retort-shaped organs at the bases of the stylets are masses of hypodermal cells constituting deep invaginations of the integument. Within these invaginations, which lengthen out in a circular manner during each stadium, new stylets are built up for each successive instar. In those members of the Hemiptera in which the head is deflected, the four stylets are separately coiled in the cephalic region and at a definite stage during each ecdysis the new stylets pass down to take the place of the old which are discarded at the moult.The manner of this renewal and replacement of the stylets appears to be, in many respects, unique. But, inasmuch as the stylets are simple hollow chitinous structures, they may be said histologically to bear a closer resemblance to other cuticular processes than they do to the more complex organs arising from imaginal buds during metamorphosis. An analogy to the renewal of the stylets is presented in the development of the spines and hairs of the body. The methods whereby the coiled stylets are expelled from the head are discussed.The action of the protractor muscles in the aphis and scale is altogether inadequate to explain the means of penetration into the compact tissues of woody growths. More especially is this the case where the stylets are much longer than the labium, and are, moreover, looped within the body and thus neutralize the effect of muscular action at the base. New facts are advanced to supplement those already known, and it is submitted that the stylets are propelled forward by successive short holds contrived by the joint action of the labrum and labium. In the aphis the main function of the stylet muscles is directional control of the tips, enabling a short stylet to explore a large area. The absence of this control in the scale is compensated for by the provision of longer stylets. Directional control permits of selective feeding, giving rise to a sense of taste. This is manifest in specific types of injury to the host plant and also by a modification in the insect of accessory structures of the mouth.