Dermanyssus gallinae, the poultry red mite (PRM), cause substantially financial losses to poultry farms worldwide (Sparagano et al., 2014). Spraying acaricides to poultry farms is the main strategy in the control of PRMs, but the overuse of acaricides has led to drug resistance (Wang et al., 2021). Biocontrol strategy based on pheromone is an effective and environment friendly method for mite control in crop production and tick control in livestock (Carr and Roe, 2016; Hamilton et al., 1989; Khan et al., 2008). However, the pheromone of PRMs has not been well explored. In a recent experiment, we found that PRM males were attracted to deutonymph females, indicating that deutonymph females could produce immature sex pheromones. The sex pheromones produced by female mites have been utilized to develop devices or methods for the prevention and control of mites (Carr and Roe, 2016; Gay et al., 2020). Identifying and isolating deutonymph females is crucial for further investigation of sex pheromones.

Sexual dimorphism (SD) is used to distinguish the male and female arthropods based on differences in morphology, physiology and behavior (Bekoff, 2004). The SD of adults in D. gallinae has been well known. Adult male possesses a holoventral shield, whereas the female has an epigynal shield due to the merging of the sternogenital and ventral components (Di Palma et al., 2012), but SD of deutonymphs has not been clearly described. Investigating the SD of deutonymphs will contribute to the identification of sex pheromones and the study of their biology.

The morphological traits of sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) represent two components of SD (Vujić et al., 2020). To describe SD precisely, it is necessary to analyze these two components (Berns, 2013). Researches on arachnid SD have mainly focused on their conspicuous SSD due to the distinct size differences in segments or body size between sexes, for example in spiders (Arachnida: Araneae) (McLean et al., 2018). The SSD is also found in mites, such as the hemophagous honey bee mite Varroa destructor and Tetranychus urticae Koch, which female mite has larger body size than male mite (Cagle, 1949; Rosenkranz et al., 2010). The SShD was also reported in other species of mites. In Cryptoribatula euaensis (Oribatida, Oribatellidae), the female carapace takes the semicircular form, whereas the male one is pear-shaped (Behan-Pelletier and Eamer, 2010). In octapod deutonymph of Tetranychus urticae Koch, males can be easily distinguished from females due to the former's smaller size and wedge-shaped posterior (Cagle, 1949; Laing, 1969).

Geometric morphometrics, developed in the 1980s and 1990s, has become a prevalent method for analyzing organismal shape variation (Klingenberg, 2016). This technique has been used to assess ecological shape changes and SD in Acari, such as Varroa destructor (Farjamfar et al., 2018), ticks (Josek et al., 2018), Hyalomma truncatum and H. marginatum rufipes (Pretorius and Clarke, 2000). In the present study we: (i) measure and compare the body length of D. gallinae deutonymphs; (ii) quantify shape of the deutonymph bodies by applying landmark-based geometric morphometrics to ascertain whether they have SD in body size and shape.