The Musculature of the Head of the Corydalus Larva (Neuroptera, Sialidae)

THE MUSCULATURE 0%’ THE HEAD OF THE CORYDALUS LARVA (NEURqOPTERA, SIALIDAE) 1,2 SOL KBAMER * Departmawt of Zoology, University of Wisconsm, Madison, Wisconsin ‘IWENTY-EIGIXT FIGURES The Neuroptera have long been regarded one of the most primitive groups of the holometabolous insects. Crampton ( ’21) considered the Neuroptera an extremely important group from the standpoint of the siudy of the phylogeny of higher insects, and emphasized the primitiveness of such forms as Sialis, Corydalus, and Chauliodea among the Neuroptera (Megaloptera). Although he considered the Coleoptera the more primitive of the two orders from a consideration of the head and mouthparts, he emphasized the astonishing similarity between larval Coleoptera and Neuroptera and concluded that the Coleoptera appear to lead to the Neuroptera which in turn lead to the Mecoptera, Diptera and Siphonaptera on the one hand and to the Trichoptera and Lepidoptera on the other. More recently Chen (’46),in a study on the evolution of insect larvae based on a wide series throughout the Holometabola, selected the nenropteroid genus Corydalus as possessing the most primitive or generalized known type of larva. He desig- nated this primitive type by the term “campodeoid-polypod, ’? characterized by having three pairs of thoracic and 10 pairs of abdominal legs, the latter bearing each a vesicle and a stylus. From this study Chen concluded that all existing types of larvae have been (derivedfrom this primitive type in two ways : Corydalus cornutus L. The writer is grateful for the assistance of R. E. Snodgrass in whose laboratory at the U. S. National Museum, Washington, D. C. thi$ study was begun. * Presrnt address: Department of Histology, New York University, 209 E. 231.d Street, New York, N. Y. 1 JOURVAT, OF MORPHOLOQY, VOL. 96, NO. 1 JANUARY 1955 2 SOL KRAMER (1) by specialization (chiefly the reduction of appendages which proceeds from the posterior part of the body to the anterior), and (2) by progenesis, or curtailment of the period of embryonic growth (brought about by the precocious rupture of egg envelopes). This investigation was therefore undertaken to provide a knowledge of the musculature of an entire neuropteroid larval form considered to be of primitive origin. Studies on the thorax and abdomen will follow. Although the Corydalus larva is used by a number of universities for the study of internal insect anatomy, no detailed investigation of its musculature is available. The dissection of the musculature of the head of an insect of this size, together with the unusual insight it gives of the relationship of the nerves, tracheal tubes and mouthparts, provides the student with a remarkable anatomical adventure. Moreover, apart from their value in providing evidence of homology, the muscles provide an important basis for func- tional stu'dies of insect structure and behavior. Material and technique The study of insect musculature is simplified by the careful selection of specimens and the application of killing and preservation methods which will give the best possible differenti- ation of the muscles. In addition to being readily differentiated from other tissues in the body cavity, muscles should retain their individual identity, should be flexible enough to permit probing, and should maintain their origins and insertions on the sclerites and movable parts of the exoskeleton. The study of insect musculature always requires much additional time and tedious manipulation when any of these conditions are not effected. With some insects all of these requirements can be met by simplv dropping them in 70% alcohol, or in other cases by first dropping in steaming water (70-75"C.), cooling, and then placing them in 705>alcohol. Insects fixed this way sometimes reveal muscles in a well preserved state, upon dissection, months or even years after they have been killed. CORYDALUS LARVA - HEAD MUSCLES 3 Many insects, however, require special methods for the proper preservation of their muscles, either for reasons of their large size, or the inability of the preserving medium to quickly penetrate the cuticle and prevent consequent destruc- tive action in the body tissues. Rober ('41) in studying the musculature of the head of larval and adult Sialis fiavilatern particularly recommended the use of injections of 96% alcohol, as well as the use of either Carnoy or 80% Bouin solution for preservation. Maki ( '36), in studying the muscles of the aldult Clzauliodes fornzosan(as, preserved his specimens in a solution of 75% alcohol and 25, formaldehyde. Das ('37), in a study of the ninsculature of the mouthparts of various insect larvae throughout the holometabolous orders, stated that he fixed bis material in 90% alcohol for 3-10 days, according to the size of the larva and the nature of its food. The dissections were made in 70% alcohol and the dissected mouthparts with their muscles were stained overnight in light borax carmine. Imms ('39), taking advantage of the anisotropic character of muscles, prepared whole mounts either in cedar wood oil or Canada balsam, and examined them with the aid of Polaroid microscopy. With the nicol prisms in the crossed position, muscles of the antennae and mouthparts have the appearance of self-luminous objects in a dark field. Such small muscles as are sometimes overlooked with other techniques, are thus at once evident. An eosin-staining, oil of wintergreen clearing procedure has been previously described by Kramer ( '48) which makes it possible to clearly detect the individual muscles through the transparent body wall of such forms as house fly larvae. Further investigations concerning the effect of pre- servatives and possible staining techniques on the differentia- tion of insect muscles for anatomical studies would be desir- able. Most of the specimens used foT this study were injected with and preserved in a 55% solution of chloral hydrate, and dissected in 50% alcohol. This method gave excellent results. The use of chloral hydrate for the preservation of Corydalus larvae was first indicated by Comstock and Kcllogg ('02). 4 SOL KRAMER Doctor Clarence F;. Mickel, University of Illinnesota, who col- lectcd and generously furnished most of the specimens used in this study, modified their method of preservation by inject- ing the specimens with a hypodermic syri~ige,~instead of slitting the abdomen as Comstock and Kellogg recommended. This injection technique has the further advantage of distcnd- ing the insect and its muscles. Full-grown CorydaZus larvae may be collected underncath the rocks of swift, shallow streams during late May or early June. Specimens are more readily collected underneath rocks on shore just after the larvae begin to lcave the stream for pupation. Thcsc latter, however, f reqnently contain much tissue for the formation of the pupal stage which often inter- feres with tracing the muscles to their origins and insertions. For this reason larvae collected in the stream, during ‘early May, are prcferable for muscle study. Larvae may also be collcctcd during July, August and September, but since these insects require three years to mature (Baldaf, ’39) such sum- mer forms will not be full-grown. X7hen investigating the musculature of any insect it is well to provide oneself with ample material. A (dozen specimens is usually the very minimum requirement and one frequently finds that twice that number, or more, depending upon the dif- ficulties encountered, are neoessary. The best results are to be had by working slowly. Considerable time is required to care- > Concerning this method Dr. Mickel writes: “ We kill the specimcns in boiling water as follows: for convenience sake we take five speeiinens at a time and enclose them in a square of cheese-cloth and then ‘dunk’ tho cnelosed specimens in boiling water for eight mconds. As soon as they come out of the boiling uater at the end of that time, I take a hypodermic needle with the 5% chloral hydrate in it, insert it on the underside of any of the terminal segments of the body and inject enough to distend the speeiiiien. I have a large shallow pan filled with chloral hydrate from which I refill thc nccdlc as often as is nccessary and into which I drop the sprci- n~ensas fast as thcy arc injcctcd. It took about 34hour8 to inject the 300 specimens that I collected in June. After all of the specimens have been injected I pack them loosely in quart jars, cover them completely with chloral hydrate solution and store them in a refiigerator at about 4045°F. until they are ready to be uBcd.” Dr. Mickel recommends that this treatment be tried in studying the inter nal structures of other insects. COLLYDAL,TlS LAllVh - HEAD IKTJSCLES 5 fully tease ax-ay the iritestinal tract, fat body, tracheal tubes arid other tissues and this time is largelj- wastcd when the muscles arc subsequently dissected away in wholesale fashion. Complete familiarity with the external and internal portions of the exoskeleton, suppleiiientcd by drawings, are a prelimi- nary requisite for the designation of the origin and insertion of the muscles, and for this reason dctailed drawings of the exoskeleton have herein been included. Description of the head The head of the CorydaEirs larva (figs. I, 2) is of tlic prog- nathous type, flattened dorsoventrally. The mouthparts are broadly articiilated to the anterior region of the epicraniurn which latter is roughly squared and thickly sclerotized. The head is dark brown in color and both the dorsal and ventral surfaces are marked with dark and light patterns, or niaculae, which indicate areas of niuscle attaclirriciit (figs. 3, 4). The head cnpsule (figs. 1,Z). Posteriorly, tlic occipital open- ing of the had is bordered by the postoccipital ridge (pocr). Anterior to the postoccipital ridge on the dorsal region is the occipital region (ocp), which is demarlied from the vertex area (vx) by tlic occipital suture (OCS).

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