JOURNAL OF THE LEPIDOPTERISTS' SOCIETY Volume 14 1960 Number 2 A STUDY OF FIRST INSTAR LARVft OF THE SATURNIIDft, WITH SPECIAL REFERENCE TO NEARCTIC GENERA by ROGER W. PEASE, JR. INTRODUCTION First instars of many species of Lepidoptera have been described, usually with non-comparative and non-technical descriptions. Occasionally it has been done with an eye toward interpreting structures of the mature larvre and correctly homologizing them with those of other groups. This has been singularly successful, since the first instar represents a link with the past in that primitive characters may occur in this stage which are completely modi­ fi ed after the first molt. This p<l!per will consider first instar larvre in their own right with only passing reference to other stages of development. Such concentration may seem arbitrary until an attempt is made to classify first in­ star larvre using a key constructed for mature larvre. The number and degree of structural changes taking place during the first molt require that a separate study be made of the first instar just as exclusive compaTative studies have been made of adults, pupre, and later instar larvre. The original intention was to include only the Nearctic genera of the Saturniidre, but a few species from other areas were available and since their structures suggested some interesting problems, they are included. The sub­ families represented are the Saturniinre, Hemileucinre, Citheroniinre, Rhescyn­ tidinre, and Agliinre. There are about as many systems of setal nomenclature as there have been workers in the field. Only a few have been applied to the first instar larvre of the Saturniidre. PACKARD (1905,1914), following Dy AR. used numbers to identify a few setre, numbering them from the dorso- to ventromeson. He depicted first instar larvre of many species of Saturniida~ in color and by line drawings. Unfortunately, setre are omitted from many of the color figures, and some plates do not agree with descriptions in the text (c.g., Coloradia pandora). Abdominal segments eight, nine, and ten are also sometimes con­ fused. On the other hand, the line drawings, which are Dr. PACKARD'S own, agree more closely with actual specimens. As a means to quick visual identi- 89 90 PEASE: First instar larva: Vo1.14: no .2 fication of larvee, the color figures are, of course, unsurpassed, but they should not be considered structurally complete. FRACKER (1929) constructed a key to the mature Iarvee of North American Saturniid~ using his system of Greek letters to identify setee. Char­ acters used in his key will sometimes hold for the first instar. Particularly important are the relationships of the most dorsal set~ on segments eight and nine of the abdomen. In the following discussion the setal nomenclature of GERASIMOV (1935) as modified by HINTON (1947) is followed as closely as possible. The almost mathematically precise definitions of groups of setee could be a.pplied to the Satu rniidee even though this family was not included in HINTON'S paper. The notations of FRACKER, FORBES, or HEINRICH are alternate systems. For comparisons and synonymy see HINTON (1947) and PETERSON (1948). Those setee which occur throughout the Lepidoptera in the first instal' are called primary setee. Secondary setee appear in succeeding instal'S but are usually absent in the first. It is an indication of advancement in the Satur­ niidee that some of them have numerous secondary seta~ in the first instar. HINTON recognized two types of setee: long or tactile setee and microscopic or proprioceptor setee. The former are generally distributed over the body, while the latter are restricted to areas where overlapping of cuticle is likely to occur, such as at the juncture of segments. Most of the microscopic setae on specimens preserved in alcohol were not clearly visible with the resolution and magnification (150X) used. Except for a single series of aberrant H emi­ feura larvae, the microscopic setee will be omitted from this discussion. Characters such as crochets, head structures, and thoracic legs are prob­ ably as diagnostic as body setee, but they have not been so widely used and are less easily studied. Some reference will be made to the crochets on the prolegs. All structures except the most prominent may be ruined if specimens preserved in alcohol were ever allowed to dry out. The more conspicuous seteeare usually in satisfactory condition even in some of the most severely shrunken specimens. It is hoped that the key presented here will eventually be extended to include all the prominent structures of first instal' larvae. A few terms need to be redefined for use with the first instar. If a seta is referred to as PRIMITIVE, it consists of a single stiff filament rising di reetly from the body wall. Presumably, this is the ancestral form. Chalazee are extensions of cuticle bearing one to several setee. FRACKER (1929) de­ fines scolus as a "spinose projection" of the body wall, with reference to the Saturniidee. The scali of later instars, however, ,are homologous to structures in the first instar which are clearly divided into a chalaza and a seta. Here the term CHALAZA will be used for any projection on the body wall bear­ ing primary setee which are still distinguishable from secondary setee. SCOLDS will be reserved for simil,ar structures in the Saturniinee having secondary setee on the chalazee which cannot be separated from the primary setee. As a general rule, the setee of first instar larvee of Saturniidae never be­ come fused . It is the chalazae which are fused to a varying degree. For ex- 1960 Journal of the Lepidopterists' Society 91 ample, on the dorsomeson of the ninth abdominal segment, all Hemileucime have a single chalaza. This bears two primary setre, one from each side of the body, whose chalazre are united. Very rarely an individual larva will hav.e 'a seta split or forked, but in two individuals seen (one each of Arsenura and H emileuca), the fork occurred on only one side of the body. PRIMARY AND SECONDARY SETIE The :primary setre are divided into several groups defined by their position and recurring on each segment. They are: dorsals (D 1 and D2), subdorsals (SD1 and S02), laterals (Ll and L2), subventrals (SVI and SV2), and ventral (VI). In addition there are two extra setre (XD1 and XD2) on the cervical shield of the prothoracic segment. The homologies of the setre on the tenth abdominal segment are not clear at present. The abbreviations T-l, T-2, T-3, and A-I to A-lO will be used to indicate thoracic and ab­ dominal segments numbered from anterior to posterior. When fusion of chalazc.e takes place, it occurs first within groups of setre and then between different groups. Only the letters of the setal groups will be used to describe such united structures. For example, D would refer to a structure homo­ logous to the dorsal setre (D1 and D2) and XD-SD would be a structure made up of the two SD setre combined with the two XD setre of the pro­ thoracic segment. On the cervical shield of the prothorax (T-1) there are six primary setre. In most Saturniidre four of these are represented by two large bifurcate chalazc.e or scoli on the anterior :part of the shield and two primitive setre on the posterior margin (Fig. 1). In the Saturniinre the bifurcation has disappeared, and the two scoli bear numerous secondary setre. Anthercea and A elias show increasing degrees of fusion of the bases of the two scoli, the top being plainly double in the former and single in the latter (Figs.3 and 4). On the posterior edge of the plate, two single setre rise directly from the shield without chalazre. One is usually mesocaudad of the first chalaza, and the second is about midway between the two chalazre. Dryocampa rubicunda (Fig.2) offers a chance to observe the relationships of the setre when their chalazre are reduced and almost wholly absent. The se-tre of the more dorsal anterior pair are of subequal length, while one seta of the pair below them is longer than the other. In the higher Lepidoptera, XD1 and XD2 are of equal length and are on the anterior 'Part of the cervical shield. The subdorsal group (SD1 and SD2) is below these with SD1 longer than SD2. The dorsal group (D1 and D2) forms the remaining setre, with D1 shorter and more dorsal than D2. Therefore, in the Saturniidre the more dorsal of the two anterior pairs of setre on the cervical shield will be called XD, the more lateral SD, the more dorsal of the single setre D1, and the more lateral single seta D2. D1 and 02 vary in their length relationship. Those on T-l are of nearly equal length in most Saturniidre. In Eacles imperialis D1 is bullet­ shaped and much shorter than the normally developed D2. 92 PEASE: First instal' Jarvie Vo1.14: no.2 On the meso- and metathorax (T-2 and T-3) the dorsal group is borne on a single chalaza or scolus in all Saturniidre. On the abdominal segments D 1 is antero-mesad of D2 and is usually on a chalaza. D 1 is always present on A-I to A-9 but may be fused on A-8 across the dorsomeson, in which case it is fl.anked on either side laterocaudad by D2 (Fig. IS). D2 is present on A-I to A-9 in the H emileucinre, Citheroniinre, and Rhescyntidinre but is fused on the dorsomeson of A-9 (Fig'!8).