1 A review of adhesives for entomotaxy 1 2 Andrew R. Deans 1 3 Frost Entomological Museum, The Pennsylvania State University, University Park, PA 4 16802 USA 5 Corresponding author: 1 6 Andrew R. Deans 7 Email address: [email protected] 8 ABSTRACT 9 Insect specimens that are too small to be pinned safely are usually affixed to a piece of card on a pin 10 or to the pin itself using an adhesive. This practice has been in place for more than two centuries, and 11 >400,000 such mounts continue to be accessioned annually in collections. Entomologists appear to 12 agree on the ideal properties of adhesives used in specimen preparation—i.e., that they remain (1) 13 archival, (2) reversible, (3) easy to prepare and use, and (4) safe. There remains no consensus, however, 14 regarding which adhesives satisfy these criteria. Entomologists continue to use fixatives they were taught 15 to use (institutional inertia) or which have good initial working properties, even though their archivability 16 and reversibility have never been tested or have been shown to be suspect. Museum professionals 17 recently identified this topic, adhesives applied to natural history specimens, as one that could be informed 18 by research and knowledge from other domains. This review includes a comprehensive list of adhesives 19 used in entomotaxy, with brief summaries of their properties as examined in the contexts of archaeology, 20 paleontology, art restoration, and polymer chemistry. The general conclusion is that no adhesive has 21 the properties sought by entomologists, and several commonly used brands or classes of adhesive 22 should never be used for entomotaxy, including most clear nail polishes, shellac, and certain polyvinyl ® 23 acetate-based dispersions, like Elmer’s Glue-All . 24 INTRODUCTION 25 Adhesives play diverse, critical roles in entomological collections, especially in the preparation of small 26 specimens (Figure 1) and the repair of broken appendages. Despite the long history of these practices, 27 principally for affixing insects to card (e.g., see Donovan, 1805), and the fact that as a community we 28 create >400,000 point- or card-mounted specimens annually (Deans, 2018), there remains no robust, 29 consensus-built best practice for adhesive use in specimen preparations. Statements like this from Schauff 30 (2001, page 33; emphasis mine) abound in the literature and in online curation forums (Deans and Sandall, 31 2018): 32 The choice of the best adhesive ... may be equally important [to other aspects of double 33 mounts], but unfortunately the aging properties of various glues are not known 34 An entomologist’s choice of adhesive, therefore, is primarily influenced more by his or her training, 35 personal preference, and what’s conveniently available, than by what science reveals to be archival, 36 reversible, and safe. 37 A 2001 survey of the Society for the Preservation of Natural History Collections (SPNHC) (Cato et al., 38 2001) identified “adhesives and pointing materials for use in mounting insect specimens” as a knowledge 39 base where more research was warranted (14.0%) or where information from other domains could be 40 transferred (31.3%). No research specific to this issue in entomology has been published since that survey, 41 and it’s clear from ongoing discussions (see Deans and Sandall, 2018) that concerns are still prevalent. 42 More than 60% of respondents to a recent survey (Deans, 2018) reported that they are not satisfied with 43 the current state of knowledge of adhesives used in entomology. 44 This review is an attempt to summarize what is known about the adhesives commonly used in insect 45 specimen preparation. Most of this knowledge was accumulated by surveying literature relevant to polymer 46 chemistry, art restoration, and the preservation of objects in archaeology and vertebrate paleontology. PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27184v1 | CC BY 4.0 Open Access | rec: 11 Sep 2018, publ: 11 Sep 2018 47 For each adhesive I provide some historical context, the known advantages and disadvantages, and a 48 recommendation (“verdict”), based on available research, regarding its continued use in entomology. 49 PREFERRED ADHESIVE PROPERTIES 50 Despite much uncertainty and a paucity of evidence-based recommendations, an entomologist’s choice of 51 specimen adhesive is not arbitrary. Members of the Entomological Collections Network (ECN) recently 52 ranked properties they considered important when selecting an adhesive (Deans, 2018): 53 1. The adhesive should be archival. It won’t degrade specimen over time, leach acid, etc. 54 2. The adhesive should be reversible. It can be dissolved away or otherwise removed without altering 55 the specimen, for observation or remounting (NB: More than one respondent rated this quality as 56 equal to “archival”, in terms of importance; see also Krogmann and Holstein, 2010; Noyes, 1982) 57 3. The adhesive should be easy to prepare and use. It requires no exotic solvents or processes, stays 58 sticky when needed, dries quickly (but not too quickly), etc. 59 4. The adhesive should be safe for people to use. It doesn’t require special personal protective 60 equipment (PPE) 61 5. The adhesive should be affordable. Museums are typically on tight budgets, but this was considered 62 the least important of these options 63 Other properties mentioned as important by individual members include: clarity upon drying, having 64 anti-fungal properties, being water soluble or even water insoluble. 65 The desirable properties listed above from the ECN survey largely mirror what conservation experts 66 describe as ideal characteristics of adhesives (here unranked; summarized from Horie, 2013; Down, 67 2015a): 68 • The adhesive must remain stable over time. Avoid products with plasticizers. Watch out for 69 yellowing, changes in pH or flexibility, and other signs of aging 70 • Be removable, with an explicit, documented procedure. (Most entomologists seek an adhesive that 71 can be reversed by dissolving it in water and/or ethanol; see Deans, 2018; Krogmann and Holstein, 72 2010; Noyes, 1982) 73 • Adhesive bonds with glass transition temperatures near room temperature (i.e., they are close to 74 transitioning to a rubbery state near 20°C), like polyvinyl acetate, should be kept away from dust or 75 other debris and mechanical stress. Adhesives with relatively high glass transition temperatures 76 (see shellac below) should not be used 77 • Adhesives that react with object’s surface, in this case insect cuticle and card stock or pin, should 78 not be used 79 • Adhesives that shrink extensively upon setting should not be used 80 • Upon setting the adhesive should be slightly weaker than the object it is applied to 81 • Consider the ways these products will act on the specimen beyond adhering it to card stock. Is it 82 acceptable to have the product enter the specimen through orifices? Is it acceptable to have the 83 specimen partially coated with the adhesive? 84 • Be sure to record the name of the adhesive, the declared composition, the supplier, manufacturer, 85 whether it’s been modified, etc. The formulas of commercial adhesives change over time, sometimes 86 dramatically, and researchers in the future need to know how best to interact with the specimens 87 prepared using your preferred adhesive. 88 Horie (2013) and Down (2015a) describe other important details about adhesives used in museums, 89 but those listed above are perhaps the most salient points for the entomologist. 2/15 PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27184v1 | CC BY 4.0 Open Access | rec: 11 Sep 2018, publ: 11 Sep 2018 90 ADHESIVES USED IN ENTOMOLOGY 91 The following summaries are arranged roughly in chronological order—i.e., according to when we 92 understand these adhesives entered common use in entomology. 93 1. Plant-derived gums (Arabic and tragacanth) 94 History. Many plant species, especially Fabaceae, produce exudates that are water-soluble and have 95 broad applications in the anthropogenic world (Williams and Phillips, 2000; Weiping, 2000). Gum arabic, 96 from Acacia spp., and gum tragacanth, from Astragalus spp., have a long history in entomology as 97 specimen adhesives (Donovan, 1805; Ingpen, 1827; Ingpen et al., 1839; Anonymous, 1870; Riley, 1892; 98 Tillyard, 1926; Verdcourt, 1946; Walker et al., 1999) and components of mounting media (gum arabic in 99 Hoyer’s; Brown, 1997). The gums would often be mixed with sugar, flour, isinglass, ox gall (cow bile), 100 corrosive sublimate (mercuric chloride), carbolic acid (e.g., in Leprieur’s gum), and/or ethanol or other 101 additives. A recent survey of the ECN (Deans, 2018) revealed no active users of plant-derived gums. The 102 use of gums in entomology likely dwindled with the advent and rise of manufactured resins, like polyvinyl 103 acetate (see below). Plant-derived gums are still widely used in art, as a binder for paints and pigments, 104 and as adhesives for some papers (e.g., stamps). These gums are not widely used in conservation (AIC 105 Wiki contributors, 2018). 106 Advantages. Gum arabic and gum tragacanth are safe, edible even, at least by themselves, and generally 107 remain soluble in water, even after decades. There are anecdotal reports that at least some gum preparations 108 remain stable, even after six decades of aging (Walker et al., 1988, citing formula used by Tillyard, 1926). 109 Disadvantages. These natural products are susceptible to decay by microorganisms and environmental 110 fluctuations, especially humidity. They are relatively weak adhesives and generally remain slightly acidic. 111 Riley (1892) refers to gum-based adhesions becoming brittle over time and acknowledges that they are 112 susceptible to damage by mites. Gum quality depends on many factors, including plant age and health at 113 the time of gum collection (AIC Wiki contributors, 2018), which may impart undesirable qualities on the 114 resulting product.