An Apposition-Like Compound Eye With A Layered Rhabdom in the Small Diving Beetle Agabus japonicus (Coleoptera, Dytiscidae) Lei-Po Jia 1,2 and Ai-Ping Liang 1 * 1 Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China 2 College of Life Science, University of Chinese Academy of Sciences, Beijing 100101, People’s Republic of China ABSTRACT The fine structure of the compound eyes of the adult diving beetle Agabus japonicus is described with light, scanning, and transmission electron micros- copy. The eye of A. japonicus is mango-shaped and con- sists of about 985 ommatidia. Each ommatidium is composed of a corneal facet lens, an eucone type of crys- talline cone, a fused layered rhabdom with a basal rhab- domere, seven retinula cells (including six distal cells and one basal cell), two primary pigment cells and an undetermined number of secondary pigment cells that are restricted to the distalmost region of the eye. A clear- zone, separating dioptric apparatus from photoreceptive structures, is not developed and the eye thus resembles an apposition eye. The cross-sectional areas of the rhab- doms are relatively large indicative of enhanced light- sensitivity. The distal and central region of the rhabdom is layered with interdigitating microvilli suggesting polar- ization sensitivity. According to the features mentioned above, we suggest that 1) the eye, seemingly of the appo- sition type, occurs in a taxon for which the clear-zone (superposition) eye is characteristic; 2) the eye possesses adaptations to function in a dim-light environment; 3) the eye may be sensitive to underwater polarized light or linearly water-reflected polarized light. J. Morphol. 275:1273–1283, 2014. V C 2014 Wiley Periodicals, Inc. KEY WORDS: diving beetle; Agabus japonicus; com- pound eye; layered rhabdom; polarization sensitivity INTRODUCTION Ever since Exner’s (1891) comparative study of arthropod photoreceptors, apposition and superpo- sition compound eyes have been distinguished. With a pigment-free clear zone separating the dioptric apparatus from the retinal layers, the superposition eye is generally more sensitive to light than the apposition eye (Land and Fernald, 1992; Warrant and McIntyre, 1993), provided that the eye is not too small (Meyer-Rochow and G al, 2004). Although exceptions exist (see below), most diurnally active insects possess apposition eyes, while most nocturnal insects and insects occurring in light-poor habitats have superposition eyes (Goldsmith and Bernard, 1974; Horridge, 1975). Some notable exceptions are a) a few day-active insects that possess superposition eyes including moths of the hesperid genera Cephonodes and Macroglossum (Eguchi, 1982; Warrant et al., 1999), skipper butterflies (Horridge et al., 1972), and a few other diurnal insects (Warrant, 2001) or those that have evolved modifications to superposi- tion (Fischer et al., 2013) and b) a few nocturnal insects equipped with apposition eyes such as the tropical sweat bee Megalopta genalis that navi- gates and forages at night (Greiner et al., 2004; Warrant et al., 2004). Up till now, nearly all the eyes of diving beetles of the family Dytiscidae (Coleoptera) studied were shown to possess superposition eyes (Horridge, 1969; Horridge et al., 1970; Meyer-Rochow, 1973; Sbita et al., 2007). Species of the genera Cybister and Dytiscus, large diving beetles with body lengths of 20 mm or more, dominated these earlier investigations. However, little is known about the eyes of diving beetles belonging to other genera like Agabus, which is the largest genus of Dytisci- dae (with over 150 known species) and even less is known of the eyes of the smallest diving beetles. Thus, a detailed study of the compound eyes of smaller diving beetles seemed valuable. Contract grant sponsor: National Basic Research Program of China (973 Program; to A.P.L); Grant number: 2011CB302102; Contract grant sponsor: Main Direction Program of Knowledge Innovation of Chinese Academy of Sciences (to A.P.L); Grant num- ber: KSCX2-EW-G-4; Contract grant sponsor: National Natural Science Foundation of China (to A.P.L); Grant number: 31172128; 31372249; Contract grant sponsor: National Science Fund for Fostering Talents in Basic Research (Special subjects in animal tax- onomy; to A.P.L); Grant number: NSFC-J1210002. *Correspondence to: Dr. Ai-Ping Liang; Key Laboratory of Zoolog- ical Systematics and Evolution, Institute of Zoology, Chinese Acad- emy of Sciences, 1 Beichen West Road, Chaoyang District, Beijing 100101, People’s Republic of China. E-mail: liangap@ioz.ac.cn Received 1 December 2013; Revised 17 April 2014; Accepted 11 May 2014. Published online 29 May 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jmor.20300 V C 2014 WILEY PERIODICALS, INC. JOURNAL OF MORPHOLOGY 275:1273–1283 (2014)