Morphological comparisons

Aganacris nitida. —Nineteen specimens of this species were examined. This species has been reported in the literature from Bolivia, Brazil, Colombia, Guyana, and Peru (Kirby 1906, Grant 1958). Specimens from these countries were compared and found to be morphologically nearly identical across the species’ range, with no differences in features usually used to distinguish females of different phaneropterine species (shape of subgenital plate, number of spines on genal lobes of femora, and shape of lateral lobe of pronotum). There were also no differences found in color patterns or in the yellow markings found on the hindfemur. Aganacris nitida extends throughout the Amazonian basin across a broad range from Brazil and the Guianas into Ecuador and Peru. There were no data to suggest that more than one species is represented within this widely dispersed sample.

Figs 7–9.

Stridulatory file on left tegmen of Aganacris species. 7. A. insectivora [Panama], (n= 124 teeth on file). 100×. 8. A. pseudosphex [Peru], (n= 98 teeth on file). 100×. 9. A. sphex [Brazil], (n= 100 teeth on file). 100×.

Field observations of A. nitida.—Aganacris nitida (female of the ‘female species’) was observed paired with a male Aganacris species on XI. 1–18.1986 (10:45 PM) at our Earthwatch research site (Explorama Inn, Loreto Prov., Peru). Specimens were on the dorsal surface of a broad leaf of a species of Citrus, in a grove near the edge of the rainforest. Observations were made with them in the copulation posture typical of phaneropterine katydids: male with head recessed under its body, tegmina raised vertically, abdomen acutely arched ventrally, female mounting his body from behind, with mouthparts lightly touching the base of his abdomen beneath the tegmina. The tip of the male abdomen continued to reach for the tip of her abdomen for nearly 40 min, during which time a spermatophore was partially formed at the base of the ovipositor. They disengaged when a heavy rain interrupted the pair-formation process. I collected both specimens: the male was A. pseudosphex, the female confirmed to be A. nitida. An attempt to keep these individuals alive in the laboratory for possible further mating ended 6 days later when the female died. Both specimens are retained in the NMNH collection.

Table 1.

Measurements of Aganacris species, given as means and ranges, in mm.

Although a spermatophore was not passed successfully during this episode, it is highly unlikely that they were engaged in interspecific pair formation. Interspecific matings of crickets have been induced in laboratory environments, but no field observations of either crickets or tettigoniids ever have been made of interspecific matings. In those cases where interspecific matings did take place, eggs were not viable. The observation confirms that these two species, at least, are actually the same species displaying a high level of sexual dimorphism.

Aganacris pseudosphex and A. sphex.—Aganacris pseudosphex mimics species of the wasp Erimnophila (Sphecidae), commonly found throughout the Amazon region, while numerous species of glossy black predatory wasps such as Hemipepsis and related genera (Pompilidae) may act as multiple models for A. nitida throughout its range. Rehn (1918) described A. sphex from a single male from Brazil. Grant (1958) listed three additional males, one from Guyana and 2 from Brazil; he regarded A. pseudosphex and A. sphex as very similar, with minor differences in overall size and in configuration of the fastigium (which he described in A. pseudosphex to be narrower and less evenly trigonal in dorsal view). They differed in the W/L ratio of the pronotal disc (0.77 in A. pseudosphex, 0.86 in A. sphex), in the foretibia (with tympanum in A. pseudosphex more elongate relative to its width and often [but not always] with a small spine on dorsolateral margin at the distal face of the tympanal expansion). Grant also noted differences in the degree of elongation of the tenth tergite (in A. pseudosphex being more elongated and medially more emarginated), shapes of the cerci (in A. pseudosphex somewhat more curved with distal tooth less prominent), and shape of the subgenital plate (in A. pseudosphex being broader and apically more deeply emarginate).

The most significant difference highlighted by Grant (1958) involved coloration and venation of the tegmen: in A. pseudosphex the darkened distal area is smaller, covering only the distal fourth of veins M and Rs (in A. sphex Grant reported that it covers nearly half of these same veins). Furthermore, on the specimens in Grant's type series of A. pseudosphex, a small dark colored crossvein connects veins Rs and M (Figs 10, 11). All of the specimens in Grant's type and paratype series showing this venation difference were from higher elevations in central Peru. This character was reviewed again in this study: although all of the Peruvian specimens have this crossvein, specimens otherwise identifiable as A. pseudosphex from Loreto Province had wing coloration and venation more similar to A. sphex than to A. pseudosphex, while specimens listed from Brazil (Hyutanahan) also showed coloration patterns and venation that were intermediate within these two species.

Measurements and ratios made by me on all specimens of these two species overlapped, and no character could be used definitively to separate them (Table 1).

Aganacris sphex was based on a single individual from Brazil, which falls within the levels of variation of A. pseudosphex, most specimens of which were collected in Peru. Comparing the stridulatory files of these two species, I found them to be similar [A. pseudosphex—with a tooth count of 98 teeth and a spatial distribution of teeth of 96 teeth/mm (Fig. 8); A. sphex with 100 teeth and 98 teeth/mm (Fig. 9)]. Comparing these two files with that of A. insectivora from Panama, the file of A. insectivora is longer, with a tooth count of 124 teeth and a spatial distribution of teeth of 128 teeth/mm (Fig. 7).

While it is not possible to rule out that these two species are distinct from one another, evidence has been presented herein that they are in fact very similar or the same. Reviewing evidence for supposing they are conspecific: (1) they both are found within the geographic range of A. nitida — A. sphex in the eastern area (Brazil and Guyana) and A. pseudosphex (Peru, Ecuador, Colombia, and western Brazil); (2) all measurements and morphometric ratios overlap (Table 1); and (3) the one character used by Grant (1958) to separate A. sphex from A. pseudosphex — the presence in A. sphex of a spine on the ventral anterior margin of the tympanal ridge — is also found in most specimens of A. pseudosphex. This evidence leads me to consider these two species to be variants of a single species. Aganacris pseudosphex and A. sphex also should be considered conspecific with A. nitida. Since the name “nitida” takes precedence, A. pseudosphex and A. sphex herein are both designated junior synonyms of A. nitida. Aganacris insectivora and A. velutina.—In Central America two other sympatric species — A. insectivora (known from males only) and A. velutina (known from females only) — are morphologically distinct from one another, reminiscent of the distinctions found in the South American species.

Figs 10–11.

Right tegmen and posterior wing of Aganacris species, showing pigmented cross-vein between M and Rs veins in A. pseudosphex (same vein present on A. sphex but not pigmented). 10. A. sphex, 2.0×. 11. A. pseudosphex, 2.0×. (Figures modified from Grant, 1958).

Aganacris insectivora (Fig. 1) mimics Hemipepsis mexicana (Lucas) (Pompilidae) (Fig. 2) (Belwood 1988), commonly found from Mexico to Colombia, while several species of glossy black predatory wasps including Hemipepsis and related genera (Pompilidae) may act as multiple models for A. velutina.

Belwood (1990) reported effects of predation on the singing behavior of A. insectivora on Barro Colorado Island, Panama. Leafgleaning bats often use echolocation for general orientation but tend to locate their katydid prey passively by listening for the sounds they make. Four of the six katydid species taken by Micronycteris hirsuta on Barro Colorado Island in Panama call in the 23–27 kHz range, suggesting which prey are taken depends on whether the bat can hear their call or not (Belwood 1990).

The selection pressure from bats has resulted in a number of adaptations in calling behavior. Some species avoid predation by calling when bats are least active. For example, Aganacris insectivora has two active periods, one during the day and one during the night. Their mimicry of wasps protects them from daytime predators such as birds, monkeys, and lizards, but not during the night. At night, they sing in a very narrow time band between 3 and 5 AM, coinciding with a lull in feeding behavior of leaf-gleaning bats. The strategy appears to be effective because gleaning bats do not appear to take these species (Robinson and Hall 2002). It has not been demonstrated whether this singing cycle also occurs in South American species, but in northern Peru leaf-gleaning bats are present in rainforests inhabited by A. pseudosphex and are most active in the early evening (9 PM–12 AM) (Belwood, pers. observ.).

Based on information that the two South American [male] species (A. pseudosphex and A. sphex) are conspecific with the sympatric species known only from females (A. nitida), it is expected that sexual dimorphism also occurs in the other species of Aganacris. Material at the Instituto Nacional de Bioversidad [INBIO] in Costa Rica and at the Smithsonian Tropical Research Institute [STRI] in Panama encompasses 35 specimens of A. insectivora and 27 specimens of A. velutina, with no specimens of the opposite sex found for either species, even though Costa Rica and Panama have been extensively collected for tettigoniids as part of ongoing biodiversity studies. It is unreasonable to expect that the opposite sex for both of these sympatric species should be so elusive as to avoid capture by trained collectors quantifying biodiversity at the several research sites in Costa Rica and Panama. It is therefore probable that these too represent a single dimorphic species. Since the name “velutina” takes precedence, A. insectivora is designated herein the junior synonym of A. velutina.

A summary list follows: