Coexistence mechanisms for species competing for the same resource include resource partitioning, neutrality, microhabitat preference, and trade-offs between competitive and dispersal abilities. We explored the coexistence mechanism of two species of trigonidiid crickets (Dianemobius nigrofasciatus and Polionemobius taprobanensis) that share the same habitat. Dianemobius nigrofasciatus is more common in areas where the ground surface is somewhat open, while P. taprobanensis is more common in more densely vegetated environments. The effects of micro-environmental differences, similarities in competitive ability, and investment in dispersal ability under interspecific competitive conditions on the coexistence of these species were examined using laboratory experiments. Both P. taprobanensis and D. nigrofasciatus performed better in vegetated environments. Although the adult emergence of D. nigrofasciatus was delayed by the presence of P. taprobanensis, the emergence rate of P. taprobanensis was not significantly affected by D. nigrofasciatus. The presence of P. taprobanensis caused a higher frequency of a long-winged morph (macropterous) of D. nigrofasciatus. The results suggest that D. nigrofasciatus is inferior to P. taprobanensis in interspecific competition, and it, therefore, disperses (as macropterous adults) at greater rates in the presence of P. taprobanensis. Furthermore, it may be that D. nigrofasciatus has been forced to change its preferred microhabitat from vegetative habitats, which are inherently more suitable, to more open environments due to competition. The above mechanisms are thought to allow the two species to coexist in the same habitat.

We explored the coexistence mechanism of two species of trigonidiid crickets (Dianemobius nigrofasciatus and Polionemobius taprobanensis) that share the same habitat. Dianemobius nigrofasciatus is more common in areas where the ground surface is somewhat open, while P. taprobanensis is more common in more densely vegetated environments. The effects of micro-environmental differences, similarities in competitive ability, and investment in dispersal ability under interspecific competitive conditions on the coexistence of these species were examined using laboratory experiments.

To determine whether the density of heterospecific crickets affected survival or adult emergence rates during the nymphal stage, a laboratory experiment was carried out in plastic containers (height = 10 cm, diameter = 10cm). Newly emerged first instar nymphs were collected within 72 hours. Estimated population density in the field was 3-4 individuals/m2 (see supplemental materials), but this is the density of adults or final instar nymphs. The density of nymphs immediately after hatching is probably higher (one female of this species lays about 80 eggs). On the other hands, these crickets can grow and reproduce at fairly high densities. In previous studies, D. nigrofasciatus reared at 5.2 to 150 nymphs / L (Shimizu & Goto, 2024, Masaki 1972); P. taprobanensis can also be reared at 100 nymphs / L at high-density case (Tanaka 2022). To examine the effects of interspecific competition in this study, the experiment was conducted at densities that are assumed to be higher than in the field (i.e., 38.2 nymph / L). Treatments (numbers of crickets of each species) were as follows (no. of D. nigrofasciatus: no. of P. taprobanensis): (30:0), (0:30), and (15:15). These three treatments were crossed with two types of rearing conditions: (1) the open habitat treatment consisted of wet soil and a portion of one paper egg carton (portion for one egg) and (2) the vegetated treatment consisted of wet soil from which Japanese lawn grass (Zoysia japonica) turf grew on the whole surface (Fig. 2b,c). The lawn grass turf was collected in a sports ground at Kagoshima University. There were seven replicates for each combination of the three densities and the two rearing conditions. The containers were maintained in incubators (CDB-14LA, Daiwa Industry LTD., Osaka, Japan) at 27°C and a short-day photoperiod (12L:12D photoperiod). Under the temperature and photoperiod, half of the first instar nymphs become adults within 40 days (Masaki, 1972). The positions of all containers were rotated in the incubators every 3 or 5 days to homogenize the environmental conditions. The food was presented in two containers (1.4 cm diameter), not directly on the soil, to prevent mold. The two types of food (cat chow and guinea pig food) were powdered and mixed. Because of the small size of the food containers, it was not possible for more than one individual over 3-4 instar nymphs to feed at the same time. The food was changed every three days.

Nymphs were reared for 38 days after the start of the experiment, and the number of surviving crickets (survival rate) and their life stage (i.e., adults or nymphs) by species and sex (adult emergence rate) were recorded. The wing morph (micropterous or macropterous) of all adults was also recorded. The macropterous morph is easily determined by the hind wings being longer than the end of the body. Tibial lengths (both tibia for each individual) of adults were measured using a digital microscope (3R-MSUSB401; 3R Solution Corp., Tokyo, Japan) as an index of body size. Head width is usually used as an indicator of body size in these crickets (e.g., Matsuda et al. 2018; 2019). However, in this experiment, the entire container was frozen in order to stop the developmental stage at the end of the experiment. The heads of frozen individuals are often twisted and complex to measure accurately. Therefore, we used the tibial length, which is easy to measure accurately even in frozen individuals, as an indicator of body size.