HUMANS and NATURE, No.l, 57-62, March 1992

Unique Sensillum Structure of the Formicid Labial Palpi

()

Yoshiaki Hashimoto

Division of Phylogenetics, Natural History Museum Projects, Administration Office, Nakayamate-dori 6-1-1, Chuo-ku, Kobe, 650 Japan

Abstract Sensillum structure of the labial palpi in the Formicidae was compared with that in the other aculeate Hymenoptera. The labial palpi of the Formicidae had many setiform sensilla and a digitiform sensillum on the tip, showing the same sensilla pattern as in the Tiphiidae and the Mutillidae. Most formicids showed extreme reductions in the numbers of setiform sensilla and bristles. Some of them had elongated pegs of digitiform sensilla. These conditions were unique to the Formicidae in the Aculeata Hymenoptera.

Key words : Formicidae, sensilla, labial palpi, Aculeata

Introduction Materials and Methods

Behavioral responses in host or mate Materials: Only female specimens were selection, locomotion and defence are fre- examined in this study. In social groups quently elicited by chemical stimuli in such as the Formicidae, and . The major sensory organs for such Apidae, worker specimens were used, be- stimuli are the antennal and labial sensilla cause workers are the best known and most (Chapman, 1982). available caste in these insects. The formicids can perceive various chemi- The species examined in this study are in cal substances at lower concentrations than Table 1. the other insects (Wilson, 1971). Despite The classification of the Aculeata Hymeno- such a capability, very little attention has ptera adopted here is that of Brothers been paid to the sensillum structure in this (1974 ) ; whereas the Formicidae are classi- group of insects (cf. Hashimoto, 1990 ). fied according to Wheeler and Wheeler Especially, the structure of labial sensilla of (1985). the Formicidae has never been closely inves- SEM observation: The labial palpi were tigated. examined by scanning electron microscope The present study aims to characterize (Hitachi-Akashi MSM4C-102). The sam- structural features of the labial sensilla of ples were cleaned by an ultrasonic-washer major taxonomic groups of the Formicidae in chloroform-methanol (2:1), dried in air, by means of the observations with use of mpunted on stubs and then sputtered with SEM and to compare the basic structural gold. To show the inner aspect of the cuti- featues with those of other aculeate clur structure, the samples were cut with a Hymenoptera. razor blade and the cellular material was Table 1. List of taxa examined. dissolved with 10% KOH before the gold sensilla and no bristles on the distal segment. coating was applied. Such an extreme reduction in the numbers of setiform sensilla and bristles was found only in these formicid taxa among the Acu- Results leata. The types of labial-palp sensilla of the All the formicids examined had one digiti- Aculeata Hymenoptera form sensillum on the tip of last labial-palp The sensilla on the labial palpi are poorly segment (Figs. 10 and 12). The pegs on this known in the Aculeata in contrast to the type of sensilla are very small in the most other sensory organs. The only compre- formicids, usually less than 5 // m, and the hensive morphological and electrophysi- ratio of peg length to distal segment length ological investigations are those of White- of labial palpus did not exceed 10%. This is head and Larsen (1976a,b) and Whitehead also the case with other Aculeata, such as (1978) using Apis mellifera. These studies the Tiphiidae and Mutillidae (Fig. 9). Some indicate that most sensilla on the labial Ponerinae including the Amblyoponini and palpi are contact chemoreceptors to the Proceratiini, however, had long pegs; 8//m monitor chemical quality of food. to 14 fi m, and the ratio ranged between 17% The present study showed that the acule- and 25% (Fig. 11). The Cerapachyinae and ates showed very little variation in the struc- Leptanillinae had also relatively long pegs ture of sensillum of the labial palpi, where of 4-5 jjl m, the ratio being 14-45%. these insects usually have setiform or digiti- form sensilla on their labial palpi (Figs. 1- Discussion 4, 6-8, 13). However, the Scoliidae are unique in having coeloconic sensilla on the The labial palpi of the Formicidae have labial palpi (Fig. 5), and the Masaridae in many setiform sensilla, and one digitiform having only large bristles (Fig. 7). sensillum on the tip. It is of great interest that the same sensilla pattern is found in The sensillum structure of Formicidae the Tiphiidae and Mutillidae, which have The peculiarities found in the structure of frequently been considered to be the closest sensillum of the labial palpi of Formicidae relatives of the Formicidae (Mosley, 1938; are as follows : Wilson, 1971). Although the Scoliidae are The labial palpi of most non-formicid also considered to be a sister-group of the aculeates are densely covered with setiform Formicidae (Walther, 1984), the sensilla sensilla and bristles. This condition was pattern on labial palpi in this wasp group is maintained in the Formicinae, Pseudo- unique among the aculeates and quite myrmecinae, Myrmeciinae and Dolichoderi- different from that of the Formicidae. nae. For example, the myrmeciines had Chapman (1982) stressed that large num- 15~ 19 setiform sensilla and approximately bers of the sensilla should provide responses 400 bristles on the distal segment (Fig. 10). to a wider range of environmental stimuli. On the other hand, the Dorylinae, Cera- If this is correct, the species that have a pachyinae, Leptanillinae, Ponerinae and broad range of diet should have larger num- Myrmicinae had very smooth labial palpi bers of the labial sensilla. However, the (Fig. 12); they have less than 5 setiform Formicidae dose not fit the expected pattern. Figs. 1―7. Sensilla on the labial palpi. 1, Chrysis shanghaiensis (Chrysididae). 2, Ectemnius sp. (Spheci- dae). 3, Megachile humilis (Megachilidae). 4, Auplopus sp. (Pompilidae). 5, Carinoscolia melanosoma fascinata (Scoliidae). 6, Vespa simillima xanthoptera (Vespidae). 7, Pseudomasaris coquilletti (Masari- dae). Abbreviations used for figures are as follows, a: aperture br: bristle c: coeloconic sensillum d: digitiform sensillum s: setiform sensillum. (Scale bar= 5//m) Figs. 8― 13 . Sensilla on the labail palpi. 8 and 9, Mutilla europaea mikado (Mutillidae), 9, close-up of the tip of labial palpus; note digitiform sensillum. 10, Myrmecia gulosa (Myrmeciinae, Formicidae). 11, Pro- ceratium watasei (Ponerinae, Formicidae); close-up of the tip of labial palpus showing digitiform sensillum. 12, Cryptopone sauteri (Ponerin, Formicidae), 13, Camponotus japonicus (Formicinae, Formicidae); longitudinal section of labial palpus to show the apertures of setiform sensilla; note that the bristles have no apertures at the base. (Scale bar- 5//m) Although many formicids have a widest tion of the aculeate Hymenoptera, with variety of food among the aculeates (i.e. live special reference to Mutillidae. Univ. prey, dead remains, flower nectar, Kansas. Sci. Bull., 50, 483-648. Chapman, R. F. (1982 ) Chemoreception: the nectar secreted by extrafloral nectaries, significance of receptor numbers. Adv. honeydew secreted by insects, seeds, fungi, Physiol., 6 , 247-355. etc.), they show an extreme reduction in the Hashimoto, Y. (1990 ) Unique features of number of sensilla on labial palpi. This sensilla on the antennae of Formicidae (Hymenoptera). Appl. Ent. Zool., 25 , 491- suggests that either there is no relationship 501. between the abundance of sensilla and diet Mosley B. D. (1938 ) An outline of the phylo- breadth, or the fewer sensilla have resulted geny of the Formicidae. Bull. Soc. Entomol. in a broad diet range perhaps because the Fr., 43,190-194. assumed decrease in sensitivity allowed less Walther J . R. (1984 ) The antennal patterns of sensilla of the Plathythyreini (Ponerinae) in accurate discrimination between different comparison to those of other ants (Hym. type of food. Further study, by the aid of Formicoidea), abstract, Int Congr. Ento- electrophysiology, is needed to clarify these mol., vol 17, p 28. points and to better understand ecology and Wheeler, G. C. and Wheeler, J. M. (1985) A simplified conspectus of the Formicidae. phylogeny of this unique group of insects. Trans. Amer. Ent. Soc., Ill, 255-264 Acknowledgements Whitehead, A. T. (1978) Electrophysiological response of honey bee labial palp contact I thank to Drs. S. Momoi and T. Naito of chemoreceptors to sugars and electrolytes. Physiol. Ent., 3,241-248. Kobe University for their guidance and Dr. Whitehead, A. T. and Larsen, J. R. (1976a) M.Takeda of Kobe University for critical Ultrastructure of the contact chemorecep- reading. I am indebted to the following en- tors of Apis mellifera L. (Hymenoptera : tomologists for their offering valuable mate- Apidae). Int. J. Insect Morphol. and Embryol., 5 , 301-315 . rials and useful information: Drs. R. H. Whitehead, A. T. and Larsen, J. R. (1976b) Crozier, M. Kubota, K. Masuko, K. Ogata, Electrophysiological responses of gallal con- M. Terayama, J. R. Walther, Sk. Yamane, tact chemoreceptors of Apis mellifera to and K. Yamauchi. selected sugars and electrolytes. J. Ins. Physiol . 22,1609-1616. , Reference Wilson, E. 0. (1971) The insect societies. Belk- nap Press of Harvard University Press, Brothers, D.J. (1974) Phylogeny and classifica- Cambridge, x+ 548 p.

(Accepted February 19, 1992)