15 December 2019 Dimensions of the dust of dustywings The dimensions of the dust of dustywings (Neuroptera: Coniopterygidae) Axel Gruppe1, Sarah Bastyans, Felix Fenzl, Anja Domes & Michael Gebhardt Chair for Zoology – Entomology group, Technical University Munich, Hans-Carl-von-Carlo- witz-Platz 2, 85354 Freising, Germany 1 Corresponding author: [email protected] Received 28th October 2018; revised and accepted 10th January 2019 Abstract. The dusty cover of adult Coniopterygidae was hitherto only rarely studied. Accord- ing to three published studies, the ‘dust’ consists of wheel-like micro-particles secreted by wax- glands on the bodies of the insects. Chemically, it is composed mainly of lipids with a 24-carbon fatty acid as its main component. To elucidate the actual dimensions of the micro-particles of Conio pterygidae, we have defined six parameters to quantitatively describe shape and size of the particles, which were measured on SEM micrographs. We present data on the micro-particles found on the external surfaces of Coniopteryx pygmaea Enderlein, 1906. The shape of the parti- cles conforms to current knowledge; however we have discovered two different particle types in Aleuropteryx juniperi. Most dimensional parameters differ significantly between taxa. We con- clude that there might be a phylogenetic signal in these micro-particles, in addition to a so far unknown function. Introduction Apart from the waxy epicuticle, several insect taxa such as Aleurodidae, Aphididae, Psyllidae (Homoptera), Coccidae (Coccoidea) Coccinellidae (Coleoptera), Hymeno- ptera and Lepidoptera produce hydrocarbons covering their body or parts of it; for a review, see Howard & Blomquist (2005). These waxes are produced in cuticular wax glands. Wax glands may be organized in plates on different parts of the body. Waxy structures of most insects are filamentary or band-shaped. Apart from waxy particles, brochosomes, spherical particles in the nm-range, may cover the surface of certain in- sects, e.g., Cicadellidae (Tulloch & Shapiro 1954). However, these structures do not consist of hydrocarbons but of proteins (Smith & Littau 1960). Among the wax producing insects is also the family Coniopterygidae (Neuropteri da: Neuroptera) (Meinander 1972). All known species of the subfamilies Aleuro pterygi nae and Coniopteryginae are covered by waxy particles. Brucheiserinae, however, which is represented by very few species, lacks these particles. In contrast to most other taxa, the wax of Coniopterygidae is organized in particles with a shape resembling a car wheel- rim with fluted edges (Navone 1987; Nelson et al. 2003; Zimmermann et al. 2009). The first documentation of dustywing particles was published by Enderlein (1906), including a drawing showing ring-like particles. Navone (1987) shows SEM images of Coniopteryx haematica MacLachlan, 1868, and Conwentzia psociformis (Curtis, 1834), Proceedings of the XIII International Symposium of Neuropterology, 17–22 June 2018, Laufen, Germany105 Pages 105-111, DOI:10.5281/zenodo.3569387 Axel Gruppe, Sarah Bastyans, Felix Fenzl, Anja Domes & Michael Gebhardt also discussing their functional aspects. Nelson et al. (2003) describe the chemical composition of the waxes produced by Semidalis flinti Meinander, 1972, the process of production of the particles, and give information on their dimensions. Zimmermann et al. (2009) describe the distribution and shape of wax glands in Aleuro pteryx juniperi Ohm, 1968, and Semidalis aleyrodiformis (Stephens, 1836). They also discuss the func- tion of the particles produced by these glands. Meinander (1972) suspects that the particles were secreted throughout the greater part of the life span of the adult; from observation of individuals with very sparse particle cover in the field, however, the pro- duction seems to be limited to the first part of adult life. To our knowledge, only Navone (1987), Nelson et al. (2003) and Zimmermann et al. (2009) have published detailed studies of the particles forming the ‘dust’ of dusty- wings (Coniopterygidae). These papers also include images of the secreting glands and indicate that from one gland two particles are secreted simultaneously, due to the pres- ence of a dividing structure in the centre of the gland openings. The hypodermal wax glands are mostly located in distinct patches on the abdomen, as well as on the head and wing bases (Enderlein 1906; Meinander 1972; Navone 1987; Nelson et al. 2003). From these locations the particles are spread over the entire surface of the insect us- ing their hind legs. The eyes, however, are not covered by waxy particles. Meinander (1972) mentions that the number and distribution of glands on the bodies of the insects seems to be of systematic value. The present study quantitatively describes the dimensions of the wax particles in de- tail. In particular, our focus was on the variability of the particles within individuals and between individuals of a single species. Concluding with Meinander (1972) that varia- tion within a species should be low, thus, the waxy particles could be of systematic value. Material and methods Our target species were Coniopteryx pygmaea (Enderlein, 1906), which is the most com- mon coniopterygid species in Central European coniferous forests, and the Mediterra- nean Aleuropteryx juniperi Ohm, 1968. Specimens of C. pygmaea were collected be- tween 2016 and 2018 by sweep netting Norway spruce Picea abies in the surroundings of Freising, Upper Bavaria, Germany (48.41°N, 11.71°E, 490 m a.s.l.) and pine Pinus sylvestris near Vieste, Apulia, Italy (41.89°N, 16.45°E, 50 m a.s.l.). Aleuropteryx juniperi was collected near Le Lavandou in the departement Var, France, from a cypress (Cupres­ sus sp.) hedge (43.11°N, 6.35°E, 40 m a.s.l.). Specimens were killed with KCN and stored air dried in small vials. In the lab, the tip of the abdomen of each dried individual was dissected for species determination. Abdomina were cleared in KOH-solution at ambi- ent temperature, rinsed with distilled water and transferred to glycerin for determina- tion, following the recommendation of Aspöck (1971). Measurements of particles were taken exclusively from male specimens. After species determination, dried specimens or dissected parts of them, i.e., legs or wings, were mounted on double-sided sticky carbon tape attached to standard alumini- um stubs for scanning electron microscope (SEM) inspection (Jeol JSM-IT 100LV). The 106 Dimensions of the dust of dustywings uncoated specimens were examined at acceleration voltages of 3–8 kV. The total wing (30× magnification), in addition to 5 to 20 spots per specimen from the wing, leg or abdomen were imaged (5000×). From the images, we selected particles to take measurements of different parameters of the same particle (ImageJ, Rasband 1997–2018). The particles were quantitatively characterized by six parameters of their dimensions: length (L), inner length (iL), curl length (cL), width (W), base width (bW) and inner width (iW) (Fig. 1). Variation of parameters within C. pygmaea was compared at two different levels: i) variation within one individual: from five individuals, we measured six parameters of five particles each from the abdomen, leg and wing. Data were tested for significant dif- ferences using the Kruskal-Wallis rank sum test (R version 3.2.2, The R Core Team 2015). ii) variation between individuals: from ten individuals we measured the wing length and six parameters of ten particles on the right wing and tested them for significance using the same test. Additionally, we calculated the Spearman rank correlation between fore wing length and the different parameters to check the influence of body size on the parameters. Finally, we compared the dimension of length and width of the particles found on C. pygmaea and A. juniperi measuring five particles on the right wing of each of the five individuals. Results The wax particles produced by Coniopteryx pygmaea exhibited a shape resembling an automobile wheel-rim with fluted edges (Fig. 2). Within one species, we did not find significant differences between the dimension of the particles on different parts of the insect,i.e ., abdomen, leg or fore wing. The dimen- sions of the particles and their variation are given in Figure 3. In contrast to the lack of a significant differences between different parts of the indi- viduals, the dimensions of parameters between individuals were shown not to be identi- Figure 1. Definition of six parameters used Figure 2. Wax particle produced by Conio­ for the characterization of wax particles pro- pteryx pygmaea. Magnificationca 8000×. duced by Coniopterygidae. 107 Axel Gruppe, Sarah Bastyans, Felix Fenzl, Anja Domes & Michael Gebhardt cal (n = 10; Table 1). Significant differences appeared between individuals for all para- meters except width. However, none of our parameters correlated significantly with wing length as a parameter for the size of individuals (Spearman’s rank correlation; Table 1). Surprisingly, we found two types of particles differing in ‘length’ and ‘width’ inAleuro­ pteryx juniperi, a smaller one as compared to C. pygmaea and a larger one. The latter was roughly twice the size of the smaller particle with no overlap (Fig. 4). Discussion The dust of dustywings is a well-known phenomenon in Neuropterology. This is be- cause of the change in appearance of the same insect from a whitish colour in life to Figure 3. Dimensions of six parameters used for the characterization of wax particles produced by Coniopterygidae, measured on particles of Coniopteryx pygmaea (n = 25). 108 Dimensions of the dust of dustywings Table 1. Comparision of the six parameters of particles on the fore wing produced by ten indi- viduals of Coniopteryx pygmaea (n = 10). inner curl base inner Parameter length width length length width width Kruskal-Wallis-Test, significance < 0.001 0.006 < 0.001 0.088 0.006 0.002 Spearman’s rank correlation, 0.250 0.235 -0.167 0.021 0.074 0.096 particle ~ wing length hyaline when preserved in alcohol. Even in the early literature the dust is mentioned.