Morphology and Evolution of the Insect Head and Its Appendages

SMITHSONIAN MISCELLANEOUS COLLECTIONS VOLUME 81, NUMBER 3 MORPHOLOGY AND EVOLUTION OF THE INSECT HEAD AND ITS APPENDAGES BY R. E. SNODGRASS Bureau of Entomology (Publication 2971) CITY OF WASHINGTON PUBLISHED BY THE SMITHSONIAN INSTITUTION NOVEMBER 20, 1928 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOLUME 81, NUMBER 3 MORPHOLOGY AND EVOLUTION OF THE INSECT HEAD AND ITS APPENDAGES BY R. E. SNODGRASS Bureau of Entomology S*^li(^,5"^' [is: w. 'o^P^ ik7l Zl (Publication 2971) CITY OF WASHINGTON PUBLISHED BY THE SMITHSONIAN INSTITUTION NOVEMBER 20, 1928 BALTIMOriE, MD., V. S. A. MORPHOLOGY AND EVOLUTION OF THE INSECT HEAD AND ITS APPENDAGES By R. E. SNODGRASS Bureau of Entomology CONTEXTS PACK Introduction 2 I. Evolution of tlie arthropod head 2 Cephalization 3 Development of the body iit segmented animals 12 The protocephalon 19 The definitive arthropod head 27 II. General structure of the insect head 33 The head capsule 34 The labrum and epipharynx 41 The stomodeum 42 The hypopharynx 45 The tentorium 50 III. The head appendages 56 The antennae 57 The postantennal appendages 59 The gnathal appendages 60 The mandibles 62 The first maxillae 74 The second maxillae 77 Morphology of the gnathal appendages 79 IV. Summary of important points 9° V. The head of a grasshopper 94 Structure of the cranium 95 The antennae 99 The mandibles ^0° The maxillae 102 The labium 106 The preoral cavity and the hypopharynx 107 The stomodeum 112 The mechanism for moving the head 118 VI. Special modifications in the structure of the head 120 Modifications in the fronto-clypeal region 120 Modifications in the posterior ventral region of the head 125 VII. The head of a caterpillar 131 Structure of the head capsule • ^3~ The antennae ^37 The mandibles ^38 The maxillae and lal)ium I39 The stomodeum ^45 The musculature of back of head, and nature of insect neck 150 Abbreviations used on the figures 153 References ^55 Smithsonian Miscellaneous Collections, Vol. 81, No. 3 2 SMITHSOMAX MISCKLLANEOLS CULLECTlUXS VOL. 8l INTRODUCTION Tt is regrettable that we must arrive at an understanding of things by way of the human mind. Lacking organs of visual retrospection, for example, we can only hold opinions or build theories as to the course of events that have preceded us upon the earth. Knowledge advances by what biologists call the method of trial and error, but the mind can not rest without conclusions. Most conclusions, therefore, are premature and consequently either wrong or partly wrong, and, once in every generation, or sometimes twice, reason back tracks and takes a new start at a different angle, which eventually leads to a new error. By a zigzag course, however, progress is slowly achieved. Error, then, is a byproduct of mental growth. It is not a misdemeanor in scientific research unless the erring one clings to his position when he should see its weakness. It is better to write beneath our most positive contentions that we reserve the right to change of opinion without notice. The reader, therefore, should not take it amiss if he finds certain conclusions drawn in this paper that do not fit with former statements by the writer, for no apology will be ofifered. I. EVOLUTION OF THE ARTHROPOD HEAD The head, as a dififerentiated region of an animal, is a more ancient structure than is any other specialized part of the body, and a proper understanding of the head structure involves an examination of the evidence of cephalic evolution from the very earliest period when evidence of head development can be found. Most of the Arthropoda have well developed heads, and that the arthropod head is a specialized body region, just as is the thorax or the abdomen in forms where these body regions are difi:'erentiated, is shown by the fact that in the embryo it consists of a series of body segments. In most cases, and particularly in insects, however, the head differs from the other body regions in that its component segments become so thoroughly consolidated in the adult as to leave little evidence of the primitive elements in the head structure. Even in the ontogenetic record the true history of the head development is so oljscure in many respects, and so much deleted in the early passages, that, though all the facts of embryology were known, it is probable that the assembled information would still give but an incomplete account of the phylogenetic evolution of the head. It is only by a comparative study of the head structure and its development in the various arthropod groups, and by an effort to correlate the known facts of arthropod organization with what is known in other animals successively lower in the scale of evolution, that we SNODGRASS NO. 3 INSECT HEAD by which the may arrive at a satisfactory conclusion as to the steps we must complex head of an insect has been evolved—and even then allow much for errors of judgment. CEPHALIZATION the numerous groups of meta- It has been but little questioned that resemliling the 1)lastula of zoic animals are derived from a creature embryonic blastula is exem- embryonic development (fig. i A). The early stage of the free-swimming plified, among living animals, in the planula develops larval'planula of the Coelenterata (fig. 2 A). The Bid in general embryonic development. Fig. I. —Typical early stages of a blastoderm (SW) surround- A hlastula stage diagrammatic, consisting in development of a chUon cavity" (BIc). B, C, D,. stages ing a blSocoele' gastruia fferentiation of cells m blastula, L, Cfrotn Kowalevskv i88^) : B, d (tud), and opemng icoele cavit; (GO. lined with endoderm ^,on 7oS gast mesoderm layers later stage, showing ongm of through blaftfpore (Bp) ; D, (Msd) just within lips of blastopore. the form of a hollow mass directly from the coelenterate egg, and has cilia. is covered with vibratde of cells the outer surface of which through the water The uniform motion of the cilia propels the animal (fig. and thereby one end in the direction of one axis of the body 3), the opposite as posterior. The creature is distinguished as anterior and though as yet there may be no thus becomes uniaxial and bipolar, functional differences at the differentiation of body structure. The the course of the subse- two poles of the body, however, determine Structural dififerentiation quent development of physical characters. in usual progression is called of the end of the body that is forward of evolving a head. cephalization, a term meaning the process SMITHSONIAN MISCELLANEOUS COLLECTKJNS VOL. The body of the planula is usually larger at the anterior end (figs. 2,3), and only in this does the planula attain cephalization in the Strict sense. Its principal structural dififerentiation occurs at the posterior pole, where there takes place an ingrowth of cells (fig. 2 B-D) that soon fills the hollow of the body, and finally, by the appearance of a cavity within its mass, becomes the wall of the stomach of the mature animal. The process of forming a primitive stomach, or archenteroii. as it takes place in the planula, is typified by that of gastrulation in ordinary embryonic development (fig. i A-D). The planula, of course, is a specialized larval form, and its manner of cndoderm formation can not be taken as showing how the archenteron was evolved, but the free-swimming planula does show that the primitive mouth, or hlasfoporc (fig. jC,D,Bp), was formed at the A B C D Fig. 2.—Formation of the endoderm in a coeleiiterate planula larva by pro- liferation of cells from posterior pole. (From Hatschek, 1888, after Claus.) Blc, blastocoele ; Pld, blastoderm ; Bed, ectoderm ; End, endoderm. posterior pole of the body, and not at the anterior pole. It is interest- ing to note, therefore, that the position of the mouth opening was not necessarily a primary determining factor of cephalization ; the practical site for a mouth in a free-swimming, ciliate animal was determined by the direction of the animal's movement. Korschelt and Heider (1895) have stated, if a monaxial, heteropolar planula is allowed to swim through water containing particles of carmine, it can be seen that the particles are rej^ulsed at the anterior and lateral parts of the body, but that they accumulate at the posterior pole. '' Here accordingly," say Korschelt and Heider, " was a favorable place for the reception of particles of food, and by a flattening or shallow invagination of the posterior pole these favorable conditions were increased. The archenteron, therefore, in its earliest beginnings was a pit in which to catch particles of food." NO. 3 INSECT HEAD—SNODGKASS 5 This is a satisfactory explanation of the origin of the gastrula if not questioned too closely; but Bidder (1927) rather disturbs the " idea with his statement that the laws of viscous matter make it clear that the free-swimming gastrulae we observe as larvae could never earn their own living, since the stream-lines would carry every particle of food outside the cone of dead water which is dragged behind the gastrula mouth." On the other hand, Bidder admits, '" creeping planulae or gastrulae might pick things up." A creeping animal, however, would never in the first place develop a mouth at the rear end of the body. What we want is an explanation of the original posterior position of the blastopore, and if none offered will sufifice. we must be content with the fact. The further history of the coelenterate larva has no bearing on the evolution of insects, for the creature soon becomes attached by its head end, and. probably as a result of the sedentary, plant-like habits Fig.

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