Interspecific competition leads to more long-winged morphs in two sympatric cricket species

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.

Methods
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.

物种争夺同一资源时的共存机制包括资源划分、中性理论、微生境偏好，以及竞争能力与扩散能力间的权衡关系。本研究针对共享同一生境的针蟋科（Trigonidiidae）两种蟋蟀——黑带黛蟋（Dianemobius nigrofasciatus）与锡兰拟毛蟋（Polionemobius taprobanensis）——的共存机制展开探究。黑带黛蟋多见于地表相对开阔的区域，而锡兰拟毛蟋则更偏好植被更为茂密的生境。本研究通过室内实验，探讨了种间竞争条件下微生境差异、竞争能力相似性以及扩散能力投入对这两个物种共存的影响。

两种蟋蟀在植被覆盖的生境中生长表现均更佳。尽管锡兰拟毛蟋的存在会延迟黑带黛蟋的成虫羽化，但黑带黛蟋对锡兰拟毛蟋的成虫羽化率并无显著影响。锡兰拟毛蟋的存在会提升黑带黛蟋的长翅型（macropterous）个体比例。研究结果表明，黑带黛蟋在种间竞争中弱于锡兰拟毛蟋，因此在后者存在的情况下，黑带黛蟋会以更高比例产生长翅型成虫以进行扩散。此外，由于种间竞争压力，黑带黛蟋可能被迫将其偏好的微生境从原本更适宜的植被生境，转变为更为开阔的环境。上述机制被认为可促成这两个物种在同一生境中共存。

材料与方法
本研究针对共享同一生境的针蟋科两种蟋蟀——黑带黛蟋与锡兰拟毛蟋——的共存机制展开探究。黑带黛蟋多见于地表相对开阔的区域，而锡兰拟毛蟋则更偏好植被更为茂密的生境。本研究通过室内实验，探讨了种间竞争条件下微生境差异、竞争能力相似性以及扩散能力投入对这两个物种共存的影响。

为探究异种蟋蟀的密度对若虫阶段存活率与成虫羽化率的影响，本研究在塑料容器（高10 cm，直径10 cm）中开展室内实验。实验所用的初孵1龄若虫均采集自孵化后72小时内的个体。野外实测的种群密度为3~4只/㎡（详见补充材料），但该密度对应的为成虫或末龄若虫；而刚孵化的若虫种群密度通常更高（单头雌性个体可产卵约80枚）。不过，这类蟋蟀可在较高密度下完成生长与繁殖。既往研究显示，黑带黛蟋的饲养密度可达5.2~150头/L（Shimizu & Goto, 2024; Masaki 1972）；锡兰拟毛蟋也可在高密度条件下以100头/L的密度饲养（Tanaka 2022）。为探究本研究中的种间竞争效应，实验设置的密度高于野外实际密度（即38.2头/L）。实验处理（各物种蟋蟀的数量）如下（黑带黛蟋数量:锡兰拟毛蟋数量）：(30:0)、(0:30)与(15:15)。上述三种处理分别与两种饲养条件组合：(1) 开阔生境处理：铺有湿润土壤，并放置1份纸蛋托（单份蛋托对应1个栖息位点）；(2) 植被生境处理：铺有湿润土壤，整个表面覆盖结缕草（Zoysia japonica）草坪（图2b、c）。结缕草草坪采集自鹿儿岛大学运动场。三种密度与两种饲养条件的每种组合均设置7个重复。饲养容器置于恒温培养箱（CDB-14LA，Daiwa Industry LTD.，日本大阪）中，培养条件为27℃、短日照光周期（12L:12D）。在该温光条件下，约半数1龄若虫可在40天内羽化为成虫（Masaki, 1972）。实验期间每3~5天转动一次所有容器的位置，以均衡环境条件。食物放置于两个直径1.4 cm的容器中，而非直接置于土壤表面，以避免食物发霉。食物为混合研磨的猫粮与豚鼠粮。由于食物容器尺寸较小，超过3~4龄的若虫无法同时取食。食物每3天更换一次。

实验开始后饲养若虫38天，记录各物种、各性别的存活蟋蟀数量（存活率）及其发育阶段（成虫或若虫），即成虫羽化率。同时记录所有成虫的翅型（短翅型（micropterous）或长翅型（macropterous）），长翅型个体可通过后翅长于体末的特征轻松辨别。使用数码显微镜（3R-MSUSB401；3R Solution Corp.，东京, 日本）测量成虫的双侧胫节长度，以此作为体型大小的指标。这类蟋蟀通常以头宽作为体型大小的指示指标（如Matsuda et al. 2018; 2019），但本实验在实验结束时将所有容器内的个体冷冻以固定其发育阶段，冷冻个体的头部常发生扭曲，难以准确测量。因此，本研究选用易于在冷冻个体上准确测量的胫节长度作为体型指标。