Description of the Larva of Ischnura gemina with a ...

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Aug 27, 1978 - ':f from Glen Park (same data) were also exam- ined. Diagnosis. Male larvae are easily distinguished from all other. Ischnura species by the ...
Description of the Larva of Ischnura gemina with a Key and New Characters for the Separation of Sympatric Ischnura l Larvae 2 ROSSER W. GARRISON Calle Iris UU18B, Rio Piedras, Puerto Rico, 00926 ABSTRACT Ann. Entomol. Soc. Am. 74: 525-530 (1981) of Ischnura gemina (K~nn~dy) is des.cribed, figure~,

The final-stage larva and compared with larvae of I. cervula Selys, I. denllcolhs (Burmeister), I; erratlca Calvert, and I. perparva Selys. Males are distinguished. by the shape and .s~ze of the pharate caudal appendages (cerci), which are more reliable than the trad.ltlOnally used ch~r­ acters of the prementum a~d caudal gills. F~IT.Iale larvae of all species ex~ept.l. errat~ca have similarly shaped cercI and are more difficult to segregate. A c~mbmatlOn of gl1l2 prementum, and antennal chara.cters is used to separate females. Figures of the cerCl for male and female larvae are mcluded. The proper identification of odonate larvae, as with other freshwater invertebrates, is a necessary first step in the prosecution and evaluation of aquatic ecological studies. Identification of larvae of the genus Ischnura has been difficult due to the lack of adequate reared material and the extreme similarities between species. Walker (1953) was unable to construct a key to the Canadian species, and identification of skins from reared specimens, using the keys of Smith and Pritchard (1956), has resulted in large numbers of misidentified specimens. Five species of Ischnura [I. cervula Selys, I. denticollis (Burmeister), I. erratica Calvert, I. gemina (Kennedy), and I. perparva Selys] inhabit the San Francisco Bay area, Calif. Kennedy (1915) described and figured the larvae of I. cervula and I. perparva based on about 20 reared specimens from Washington. He noted general color and leg length differences between his material when directly compared but characterized those differences as very slight, indicating that mixed series of species would result in a large number of misidentifications. Kennedy (1917) further described the larva of I. denticollis, differentiating it from the other two species by the length of setigerous margins on the dorsal gill. The large, atypical I. erratica was described, diagnosed, and figured by Cannings and Doerksen (1979). The larva of I. gemina was unknown until the rediscovery of several adult populations during 1978 and 1979 by R. W. Garrison and J. E. Hafernik, Jr. The purpose of this paper is to describe the larva of Ischnura gemina and to provide a key for the separation of sympatric species. Adequate series of reared specimens of all five species showed that traditional gill and prementum characters were, except for I. erratica, largely useless in identification. Since adult Ischnura are easily identified by the caudal appendages, these latent structures were analyzed in the larvae and were found, in the males, to provide a good means of discrimination. ~ata: Coenagrionidae. 2 Received for publication 31 October 1980.

Materials and Methods

This study is based on at least seven specimens of each species reared from at least three different localities, except for I. erratica (see species description). These samples probably encompass the scope of intra- and interdemic variation of characters. Specimens were obtained either by rearing fieldcollected larvae to maturity or by rearing larvae from eggs in isolation in plastic petri dish~s. All larvae were fed mosquito larvae. Before transformation, tripods constructed from pipe cleaners were placed in each dish. Cultures were checked every day for emergence. To obtain eggs, females were confined to plastic boxes with a moist paper towel on the bottom. The rough substrate was suitable to elicit egg laying and provided enough humidity to prevent desiccation of the adults. Smaller instars were fed various infusoria until they could eat first-stage mosquito larvae. Male larvae were identified by the shape and size of the pharate caudal appendages (cerci). These structures lie between the median and lateral gills at the end of the tenth abdominal segment (see Plate 5, Fig. 8 of Walker [1953]). Careful removal of the caudal lamellae (gills) is necessary to examine these structures. Removal of the tenth abdominal segment facilitates rotation of the cerci under the microscope. Female cerci of four species, I. cervula, I. denticollis, I. gemina, and I. perperva, are cone shaped and of little diagnostic value. Characters from the prementum, antennae, and gills were used to identify these species, but I found no single character that would easily discriminate one species from all others. Means of all characters and ratios were tested for significance by using SS-STP tests (sum of squares simultaneous testing procedures). Significance between means often showed some overlap in their ranges, and care should be used in couplets 7 through 9 of the key. Questionable specimens should be compared with the diagnoses and by association with identified males. All characters were measured to the nearest 0.01 mm by using an ocular micrometer; all illustrations were drawn with the aid of a camera lucida.

525 ©1981 Entomological Society of America

0013-8746/81/0605-2506$02.00/0

526

ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA

All specimens are in the collection of R. W. Garnson. Key to Final-Stage Ischnura Larvae of the San Francisco Bay Area

1. Sternum of abdominal segment 9 without ovipositor between lateral gonopophyses (males) .. 2. 1'. Sternum of abdominal segment 9 with ovipositor between lateral gonopophyses (females) 6. 2(1). Cercus in lateral view rounded or truncate (Fig. 3b), not slanting ventrally; in posterior view, cercus spherical (Fig. 3a); in dorsal view, cercus semicircular, about as long as wide (Fig. 3d) . . gemina. 2'. Cercus in lateral view sharply or bluntly pointed (Fig. 4b, 5b, 6b), slanting ventrally; in posterior view, cercus elongate ventrally with bluntly pointed tips converging (Fig. 4a, 5a, 6a); in dorsal view, cercus thumb shaped, about twice as long as wide (Fig. 4d, 5d, 6d) ....................... 3. 3(2'). Cercus large and robust (>0.44 mm long dorsally); dorsal margin of cercus in dorsolateral view broadly concave (Fig. 7c) ......... erratica. 3'. Cercus smaller,