Various authors have given general descriptions of the immature stages of vujous species of Stetha~cs, and Boving (1917) described certain morphological festures of the larvae of S. punctum (Leconte) and S. utilis (Horn), but no complete account of the nlorphology of these stages in any species of the genus has apparently been published. Clement (1880) gave the most thorough of the earlicr descriptions of the immature stages of S. pumcctillrtm Weise.The following descriptions are based on material reared from adults captured at Vineland Station, Ontario, where the occurrence of S. punctilluwz was recently recorded by the author (Putman, 1955). Approximately 25 individuals of each stage were examined.Egg. -Figured by ColIyer (1953). Attached to the substratum longitudinally, elongate with bluntly rounded ends, 0.22 to 0.23 by 0.34 to 0.39 mm.Chorion very finelv reticulate; white to pale yellow when fresh, becoming dusky as the embrvo de6--lops. ~o u r t h ' (last)-instar Lama.-Dark greyish to reddish-brown with darker tubercles and sclerites. Fusiform, widest on metathorax and first abdominal segment. Head (Fig. 1) prognathous, slightly deflexed. Cranium in dorsal Fig. 1. Fourth-instar larva of Stethorns punctillunt Weise; dorsal and ventral views of head and ventral view of right mandible.aspect more or less rectangular, slightly wider than long, widest near middle; sides gently curved, occipital margin nearly straight. Antero-lateral margins of cranium obliquely truncate behind to the ends of the clypeolabral suture, which is slightly convex and about two-fifths as wide as the greatest width of the cranium. Dorsum of head without distinct sutures. The membranous gular region extending nearly across the posterior width of the cranium, narrowing anteriorly t o the bases of the maxillae. Head with many long, finely pectinate setae, the longest about half as long as the width of the cranium. Labrum deflexed, broadly curved distally, more strongly rounded at sides. T w o very large lcontribution No. 3363,

Earlier work indicated that food plants of the tenthredinid sawflies attacked by Drino bohemica hlesn. and Bessa harveyi Tns. influence host-finding by these tachinid parasites (Allonteith, 1955(Allonteith, , 1958a. There is a high degree of interaction between stimuli produced by the host larvae and by their food plants (Monteith, 1955(Monteith, , 1958b.It was observed during field studies that the percentage parasjtisn~ bv B. hmveyi of the larch saxvflv, Prisriphorn erichsonii (Htr~.), varied with increasing height above the ground ind in different sections of tarnaracli trees, Lnrir lm-ici~?~~ (DuRoi) K. T

Adults of Drino bohemica Mesn., a tachinid parasite, learned to associate the movement of a part of their cage with the presence of host larvae. The parasites became habituated to an artist’s brush. Learning was retained for a period varying from a few hours to a few days. There were differences in the ability of groups of parasites to learn and to retain learning. The ability of the parasite to learn a new clue for use in locating hosts would interact with other factors that influence host-finding and host selection.

Host-finding and host selection by entomophagous insects are influenced by the plants on which their hosts feed (Milliron, 1940; Simmonds, 1944). Some species of parasites are attracted to certain plants before they receive any stimuli from the hosts (Cushman 1926; Monteith, 1955). Picard and Rabaud (1914) found that many parasitic Hymenoptera attack larvae of species of different families, and even of different orders, provided that they feed on the same species of food plant.

Two, or possibly three, types of stimuli that may influence host selection by entomophagous insects are produced by movements of the host or the prey. Movement may provide visual or tactile stimuli. Vibration caused by host movement may stimulate some species.Tactile stimuli are produced by a host if it moves when touched by a parasite and are usually provided by a host that is enclosed in a case or a cocoon. Ullyert (1936), working with Dahlbominus fuscipennis (Zett.), and Williams (1951) and Thorpe and Jones (1937), working with Idechthis canescens (Grav.), found tactile stimuli an important factor in host selection, but Simmonds (1943) and Thompson and Parker (1927) concluded it was of little importance in I. canescens and Melittobia acasta Walk. respectively. Movement of potential prey upon contact stimulates attack by foraging ants (Vowels, 1955). Tactile stimuli produced by movement of the host may indicate an unsuitable host to the egg parasite Trichogramma evanescens Westw. (Salt, 1938).