Milkweed butterflies (Lepidoptera: Nymphalidae: Danainae) are chemically defended and aposematic (Ackery and Vane-Wright 1984). The chemical ecology of several species have been studied extensively, including the iconic and migratory Monarch Butterfly, Danaus plexippus (Brower and Glazier 1975, Boppré 1993). Danaine larvae feed on chemically defended plants from which they also extract phytochemical protection. Adult males of many species supplement their existing biochemical deterrents with pyrrolizidine alkaloids (hereafter, alkaloids) obtained by scratching alkaloid-containing substrates and imbibing the resulting exudates. These alkaloids impart additional protection to males, serve as a biochemical precursor for mating pheromones, and function as a nuptial gift to females delivered in the spermatophore (Boppré 1990). Here, we report novel observations from Sulawesi of adult male danaine butterflies scratching other danaine butterfly larvae and imbibing the exudates with their proboscis, presumably to steal their host-plant-derived alkaloids.

While the use of alkaloids as chemical defenses in insects is not uncommon, it is most conspicuous in the Lepidoptera, especially in the Danainae, which comprises three tribes: Danaini, Ithomiini, and Tellervini (considered by some to be subfamilies; Lamas 2004). Danaine species use alkaloids for defense and pheromone production (Edgar and Caulvenor 1974, Boppré 1986, Orr et al. 1996). Host-plant-derived chemical defense is ubiquitous among the Danainae, and members of this group participate in diverse mimicry rings involving poisonous models and palatable non-danaine mimics wherever they occur (Parsons 1998, Joshi et al. 2017, Nishida 2017).

Pyrrolizidine alkaloids are naturally occurring hepatotoxic plant secondary metabolites commonly found in the families Boranginaceae, Apocynaceae, Asteraceae, Fabaceae, Orchidaceae, and Solanaceae (Wink 1988, 2003, El-Shazly and Wink 2014). Danaine butterflies either feed on these plants as larvae or seek and imbibe alkaloids as adults for purposes other than primary metabolism (“pharmacophagy”; Boppré 1984). In the Ithomiini, larval host plants are frequently species of Solanaceae and Apocynaceae (Trigo et al. 1996, Willmott and Freitas 2006), whereas in the Tellervini and Danaini, these are mostly plants from the families Apocynaceae and Moraceae (Ackery and Vane-Wright 1984). Adult pharmacophagy is less taxon-specific, and alkaloids are sequestered from a wide variety of sources, including flowers, damaged or withered leaves, and dried seed pods of alkaloid-producing plants (all authors personal observations; Edgar and Caulvenor 1974, Trigo et al. 1996). Plants in the genera Senecio (Asteraceae), Heliotropium (Boraginaceae), and Crotalaria (Fabaceae) are among the most popular targets, consisting mostly of the alkaloids retronecine and heliotridine (Smith and Culvenor 1980, Boppré 1990). Individual plants in the Asteraceae genus Senecio, for example, may comprise as much as 18% in alkaloids by dry mass (Molyneux and Johnson 1984).

For adult butterflies, alkaloids are used for more than just chemical defense. They are important precursors for pheromone production, particularly for the biosynthesis of danaidone and hydroxydanaidal (Schulz 1998). In danaine butterflies, males may possess two types of androconial organs: a pair of eversible abdominal hairpencils and a pair of alar glands (sex brands; Ackery and Vane-Wright 1984, Boppré and Vane-Wright 1989). During pre-copulation, males hover over receptive females with hairpencils extruded and splayed, disseminating pheromones that are detected by females (Brower et al. 1965, Honda et al. 2016). Courtship success correlates with mating success and fecundity (Edgar and Caulvenor 1974, Honda et al. 2018), and males often transfer alkaloids to females through spermatophores as nuptial gifts (Boppré 1990). While alkaloids are often ingested in leaves consumed by caterpillars during the larval stages, they may be stored as intermediary compounds (Orr et al. 1996) and are not always transferred to the adult stage (Boppré 1990). Withered and dried parts of alkaloid-producing plants are a favorite of danaine butterflies, possibly due to higher concentrations and retention of alkaloids. Fresh leaves and other vegetative parts are also a target in the absence of dried or withered leaves. Unique to the danaine butterflies is the unusual behavior known as “leaf-scratching” (Boppré 1983), in which adult butterflies damage plant parts with their sharp tarsal claws, liberating plant juices and sap to imbibe.

Rarely, danaines have been reported imbibing fluids directly from carcasses of alkaloid-storing insects, though this phenomenon has been reported infrequently. For example, in Africa, Danaus chrysippus has been observed to feed on juices of moribund pyrgormorphid grasshoppers that have previously fed on alkaloid-producing plants, apparently taking advantage of their carcasses as an alternative source of alkaloids (Owen 1971, Bernays et al. 1977). This behavior has been observed independently by the authors Y. K. Tea and J. W. Soong in Singapore. Both observations involved adults of Parantica agleoides agleoides (Nymphalidae: Danainae) feeding from dead lepidopteran carcasses, the pupa of another danaine species in one case, and a dead arctiine moth in the other (Fig. 1A). Whether the butterflies were actively imbibing alkaloids from these sources is unclear, but danaine pupae and arctiine moths are both chemically defended by alkaloids (Rothschild et al. 2008).

On 9 December 2019, however, Y. K. Tea and J. W. Soong observed adult male danaine butterflies scratching at living butterfly caterpillars in coastal forests of Tangkoko Batuangus Nature Reserve, North Sulawesi, Indonesia (1°34′33.2″ N, 125°09′36.1″ E) and apparently imbibing fluid from them. The behavior was similar to danaine leaf-scratching, but the butterflies instead scratched live Idea blanchardii blanchardii larvae that were feeding on Parsonsia spp. (Apocynaceae; Fig. 1B–E). Adult butterflies of multiple danaine species were observed scratching many caterpillars along a stretch of coastal vegetation (˜500 m long), with mid-to-late-instar caterpillars appearing to be the preferred targets. The larvae would contort their bodies rapidly in what appeared to be futile attempts to deter the persistent scratching of adults. Adult danaine butterflies were observed actively imbibing from the wounded and oozing caterpillars, sometimes in mixed species groups that would congregate for several hours (Fig. 1D). Feeding butterflies were often so engrossed that they were oblivious to being touched by human observers. Subsequent observations over three days revealed repeated visits by danaine butterflies to the same wounded larvae. Adults were also observed scratching and imbibing from dead caterpillars, although whether these mortalities were a direct consequence of harassment and scratching is unclear. Although multiple danaine species were abundant at this locale, adult butterflies were only observed scratching larvae of I. blanchardii blanchardii. No larvae of other danaine species were found. Pupae of I. blanchardii blanchardii were also observed in situ, but these remained unmolested and appeared to be of no interest to adult butterflies. We also include a photograph of Idea blanchardii blanchardii feeding on a larval carcass of its own species taken in 2017 at the same location by an amateur naturalist (Fig. 1F).

In total, we observed seven danaine butterfly species scratching larvae, both dead and alive. These were Danaus ismare alba, Euploea algea kirbyi, Idea blanchardii blanchardii, Ideopsis juventa tontoliensis, Ideopsis vitrea vitrea, Parantica cleona luciplena, and Tirumala ischmoides ischmoides. All of these species, along with adults of four other danaines (Danaus genutia leucoglene, Euploea eupator eupator, Euploea hewitsonii hewitsonii, and Euploea westwoodii meyeri), were also observed scratching leaves (Fig. 2A–L). All 11 danaine species scratched leaves intermittently during our hours of observation, between 09:00 and 14:00, mostly on lactiferous vines in the genus Parsonsia and other unidentified apocynaceous plants. Leaf-scratching was also observed on trees in the genus Premna (Lamiaceae). While species of Premna (and most other Lamiaceae) are not known to produce pyrrolizidine alkaloids, they contain isoxazole alkaloids (Barik et al. 1992) and icetexane diterpenes (Dianita and Jantan 2017). We do not know whether these compounds are attractive to danaine butterflies or play roles similar to pyrrolizidine alkaloids. Of the dozens of adult butterflies observed scratching plants or caterpillars, only one was female (Fig. 2B). Observations of females engaging in leaf-scratching are unusual, since it is almost exclusively males that engage in this behavior, often gathered in large aggregations (sometimes numbering in the hundreds) where they imbibe alkaloids from suitable plants with fervor (Fig. 2M, N).

Why danaine butterflies harass and scratch caterpillars at this locale remains unclear. Similarly, it is not known whether the scratching butterflies are attracted by the larvae, their host plants, or both. One possibility is that caterpillars feeding on apocynaceous plants at this site are a more concentrated source of alkaloids compared to their host plants (see Orr et al. 1996). In addition, the thin larval integument of Idea blanchardii blanchardii may permit easier access to alkaloid-containing exudate than through leaf or pupal scratching. The congeneric Idea leuconoe from Japan is similar in having larvae that feed exclusively on Parsonsia. Under laboratory conditions, the pyrrolizidine alkaloids ideamine A and B N-oxides have been isolated from adults, eggs, and host plants, demonstrating the acquisition and transfer of alkaloids directly from the host to the larva in this species (Nishida et al. 1991, ideamine B is a nor-derivative of parsonsine). Whether a parallel situation occurs in the Sulawesi endemic I. blanchardii blanchardii is unknown, though this could explain the preference for live and dead caterpillars of I. blanchardii blanchardii by adults of the same species, as well as those of other danaine butterflies at this locale. We note that while our observations of scratching by adult butterflies at this locale have been limited to leaves and caterpillars, we are aware of at least one incident where this behavior has been performed directly on an adult butterfly (video available online).99 https://doi.org/10.6084/m9.figshare.16571115.v1 The video, taken at the same locale in 2017 by Samaisukh Sophasan, clearly shows a male Ideopsis vitrea vitrea harassing, scratching, and probing the wings of an adult I. blanchardii blanchardii with its proboscis.

The population density of danaine butterflies in the vicinity may also contribute to more frequent observations of this previously unknown phenomenon. The coastal habitat of Tangkoko Batuangus Nature Reserve in Sulawesi is bordered by dense coastal vegetation that is replete with Apocynaceae. Species of Danaini were the dominant group of diurnal Lepidoptera, particularly those of the genera Danaus, Euploea, Idea, Ideopsis, Parantica, and Tirumala. One alternative hypothesis is that the butterflies are attracted to the damaged leaves left by feeding caterpillars, and as a result are in close proximity to caterpillars that they inadvertently wound through leaf-scratching. The wounded caterpillars and their alkaloid-rich exudates then act as a beacon, drawing the attention of butterflies already in the vicinity. Continuously overlapping generations in this tropical habitat may also allow newly eclosed adult butterflies to observe and learn the larval scratching behavior from older individuals, resulting in cultural transmission of this unusual behavior (Whiten 2019). Although we report the occurrence of 11 species, seven of which were observed feeding on caterpillars, it is likely that this list will increase with additional observation.

As far as we are aware, these are perhaps the first observations of milkweed butterflies apparently obtaining alkaloids from the caterpillars of living, closely related species rather than plants or the dead carcasses of insects from different orders. We presume that alkaloids are transferred from the larvae to the adults, but this hypothesis requires chemical verification, and we therefore exercise caution interpreting our observations. Since the larvae appear to be unwilling participants in this presumed exchange, we suggest this behavior be termed “kleptopharmacophagy,” which highlights that the chemicals consumed by the adults are stolen from others. However, since it is not clear whether the caterpillar wounds and fatalities were caused by scratching, the alternative neologisms “kairopharmacophagy” (feeding on defensive chemicals from wounded caterpillars detected via “eavesdropping”) or “necropharmacophagy” (feeding on defensive chemicals from dead caterpillars) might also be appropriate. It is not known whether this behavior occurs elsewhere in the tropics or is truly restricted to this region. Danaus species of the Americas, for example, are apparently not known to perform leaf-scratching (Ramos et al. 2020, Lawson et al. 2021). It is possible that the dearth of natural history observations on Sulawesi may have precluded observations elsewhere on the island (but see Tea et al. 2020).

乳草蝴蝶（鳞翅目：蛱蝶科：Danainae）具有化学防御性和无性（Ackery and Vane-Wright 1984）。几个物种的化学生态学已被广泛研究，包括标志性和迁徙的帝王蝶，Danaus plexippus（Brower 和 Glazier 1975，Boppré 1993）。Danaine 幼虫以化学防御植物为食，它们也从中提取植物化学保护。许多物种的成年雄性用吡咯里西啶生物碱（以下简称生物碱）来补充它们现有的生化威慑力，这种生物碱是通过抓挠含有生物碱的底物并吸收产生的渗出液而获得​​的。这些生物碱为雄性提供额外的保护，作为交配信息素的生化前体，并作为对在精囊中分娩的雌性的结婚礼物（Boppré 1990）。在这里，我们报告了来自苏拉威西岛的成年雄性达那因蝴蝶抓挠其他达那因蝴蝶幼虫并用它们的长鼻吸收渗出液的新观察，大概是为了窃取它们的寄主植物衍生生物碱。

虽然在昆虫中使用生物碱作为化学防御剂的情况并不少见，但在鳞翅目中最为显着，尤其是在 Danainae 中，它由三个部落组成：Danaini、Ithomiini 和 Tellervini（被一些人认为是亚科；Lamas 2004）。Danaine 物种使用生物碱来防御和产生信息素（Edgar 和 Caulvenor 1974，Boppré 1986，Orr 等人1996）。寄主植物衍生的化学防御在 Danainae 中无处不在，该组的成员参与各种模拟环，包括有毒模型和可口的非 danaine 模拟物（Parsons 1998，Joshi 等人2017，Nishida 2017）。

吡咯里西啶生物碱是天然存在的具有肝毒性的植物次生代谢产物，常见于紫草科、夹竹桃科、菊科、豆科、兰科和茄科 (Wink 1988 , 2003 , El-Shazly and Wink 2014 )。达那因蝴蝶要么在幼虫时期以这些植物为食，要么在成年后寻找和吸收生物碱，用于初级代谢以外的目的（“药理学”；Boppré 1984）。在 Ithomiini 中，幼虫寄主植物通常是茄科和夹竹桃科 (Trigo et al. 1996 , Willmott and Freitas 2006 )，而在 Tellervini 和 Danaini 中，它们主要是来自夹竹桃科和桑科 (Ackery and Vane-Wright) 的植物1984 年）。成体药物自噬对分类群的特异性较低，生物碱可以从多种来源中分离出来，包括花、受损或枯萎的叶子，以及产生生物碱的植物的干燥种子荚（所有作者的个人观察；Edgar 和 Caulvenor 1974，Trigo 等. 1996 年）。在属植物千里光（菊科），天芥菜属（紫草科），和猪屎豆（豆科）是最流行的目标之一，主要由的生物碱retronecine和heliotridine（Smith和Culvenor 1980年，Boppré 1990年）。菊科千里光的单株植物例如，按干重计，生物碱的含量可能高达 18% (Molyneux and Johnson 1984 )。

对于成年蝴蝶，生物碱不仅仅用于化学防御。它们是信息素生产的重要前体，尤其是达那酮和羟基达那醛的生物合成（Schulz 1998）。在 danaine 蝴蝶中，雄性可能拥有两种类型的雄性器官：一对可翻转的腹部毛笔和一对鼻翼腺（性别品牌；Ackery 和 Vane-Wright 1984 年，Boppré 和 Vane-Wright 1989 年）。在交配前，雄性盘旋在有接受能力的雌性身上，头发被挤出和张开，传播由雌性检测到的信息素（Brower 等人1965 年，Honda 等人2016 年）。求偶成功与交配成功和繁殖力相关（埃德加和考尔弗诺1974 年，本田等人。2018），雄性经常通过精囊将生物碱作为结婚礼物传递给雌性（Boppré 1990）。虽然生物碱通常在幼虫阶段被毛虫吃掉的叶子中摄取，但它们可能作为中间化合物储存（Orr 等人，1996 年），并不总是转移到成虫阶段（Boppré 1990 年）。产生生物碱的植物的枯萎和干燥部分是达那因蝴蝶的最爱，这可能是因为生物碱的浓度和保留率较高。在没有干燥或枯萎的叶子的情况下，新鲜的叶子和其他营养部分也是一个目标。达那因蝴蝶的独特之处在于被称为“刮叶”的不寻常行为（Boppré 1983)，其中成年蝴蝶用锋利的跗骨爪破坏植物部分，释放植物汁液和树液以供吸收。

很少有报道称达那因直接从储存生物碱的昆虫尸体中吸取液体，尽管这种现象很少见。例如，在非洲，据观察，Danaus chrysippus以垂死的蚱蜢的汁液为食，这些蚱蜢以前以生产生物碱的植物为食，显然是利用它们的尸体作为生物碱的替代来源（欧文1971 年，伯内斯等人1977 年））。新加坡的作者 YK Tea 和 JW Soong 独立观察到了这种行为。两项观察均涉及Parantica agleoides agleoides 的成虫（蛱蝶科：Danainae）以死去的鳞翅目昆虫的尸体为食，一种情况下是另一种 danaine 物种的蛹，另一种情况下是一只死螟蛾（图 1A）。蝴蝶是否积极吸收这些来源的生物碱尚不清楚，但丹宁蛹和螟蛾都受到生物碱的化学保护（Rothschild 等人，2008 年）。

然而，在 2019 年 12 月 9 日，YK Tea 和 JW Soong 在印度尼西亚北苏拉威西岛 Tangkoko Batuangus 自然保护区（1°34′33.2″ N，125°09′36.1″）的沿海森林中观察到成年雄性达那因蝴蝶在活体蝴蝶毛虫身上抓挠E) 并且明显地从它们吸收液体。这种行为类似于danaine 抓叶，但蝴蝶却抓了以Parsonsia为食的活的Idea blanchardii blanchardii幼虫属 （夹竹桃科；图 1B-E）。观察到多种达那因物种的成虫沿着海岸植被（约 500 m 长）抓挠许多毛虫，中晚期毛虫似乎是首选目标。幼虫会迅速扭曲身体，试图阻止成虫持续抓挠似乎是徒劳的。观察到成年达那因蝴蝶积极地从受伤和渗出的毛虫身上吸食，有时在混合物种群中会聚集几个小时（图 1D）。喂食的蝴蝶常常全神贯注，以至于它们都没有被人类观察者触动。随后三天的观察显示，达那因蝴蝶会反复访问同一只受伤的幼虫。还观察到成虫从死去的毛毛虫身上抓挠和吸食，尽管这些死亡是否是骚扰和抓挠的直接后果尚不清楚。尽管该地区有多种达那因物种，但仅观察到成年蝴蝶抓挠幼虫I. blanchardii blanchardii。没有发现其他丹宁种的幼虫。在原位也观察到I. blanchardii blanchardii 的蛹，但这些蛹没有受到干扰，并且似乎对成年蝴蝶没有兴趣。我们还包括了一张Idea blanchardii blanchardii以自己物种的幼虫尸体为食的照片，该照片由一位业余博物学家于 2017 年在同一地点拍摄（图 1F）。

我们总共观察到七种达那因蝴蝶抓挠幼虫，无论是死的还是活的。这些都是斑蝶ismare阿尔巴，紫斑蝶属algea kirbyi，理念blanchardii blanchardii，Ideopsis忽闻tontoliensis，Ideopsis vitrea vitrea，绢斑蝶属cleona luciplena和蒂鲁马拉ischmoides ischmoides。所有这些物种，以及其他四种 danaines（Danaus genutia leucoglene、Euploea eupator eupator、Euploea hewitsonii hewitsonii和Euploea westwoodii meyeri）的成虫），还观察到刮伤叶子（图 2A-L）。在我们的观察时间内，所有 11 种达那因物种都在 09:00 至 14:00 之间间歇性地刮擦叶子，主要是在Parsonsia属的泌乳藤蔓和其他身份不明的夹竹桃植物上。在Premna属（唇形科）的树木上也观察到刮叶现象。虽然不知道Premna（和大多数其他唇形科）会产生吡咯里西啶生物碱，但它们含有异恶唑生物碱（Barik 等人，1992 年）和icetexane 二萜（Dianita 和 Jantan 2017 年））。我们不知道这些化合物是否对达那因蝴蝶有吸引力，或者是否起到类似于吡咯里西啶生物碱的作用。在观察到的数十只抓挠植物或毛虫的成年蝴蝶中，只有一只是雌性（图 2B）。对从事抓叶活动的雌性的观察是不寻常的，因为几乎完全是雄性从事这种行为，通常聚集成大群（有时数以百计），在那里它们以热情的方式从合适的植物中吸收生物碱（图 2M，N ）。

为什么达那因蝴蝶在这个地区骚扰和抓挠毛虫仍不清楚。同样，不知道抓挠的蝴蝶是被幼虫、寄主植物还是两者都吸引。一种可能性是，与寄主植物相比，以该地点的罗布麻植物为食的毛虫是生物碱的更集中来源（见 Orr 等人，1996 年）。此外，与通过叶片或蛹抓挠相比，Idea blanchardii blanchardii 的幼虫外皮较薄，可能更容易接触到含有生物碱的渗出液。来自日本的同类Idea leuconee 的相似之处在于其幼虫仅以Parsonsia为食。在实验室条件下，吡咯里西啶生物碱ideamine A和BN-氧化物已从成虫、卵和寄主植物中分离出来，证明了生物碱直接从寄主获得并转移到该物种的幼虫中（Nishida 等，1991，ideamine B 是帕森辛的非衍生物）。苏拉威西岛特有的I. blanchardii blanchardii是否发生类似的情况尚不清楚，尽管这可以解释对I. blanchardii blanchardii 的活毛虫和死毛虫的偏好由同一物种的成虫，以及该地区其他达那因蝴蝶的成虫。我们注意到，虽然我们在该地区对成年蝴蝶抓挠的观察仅限于树叶和毛毛虫，但我们知道至少有一个事件是直接在成年蝴蝶身上进行这种行为的（视频可在线获取）。99 https://doi.org/10.6084/m9.figshare.16571115.v1这段由 Samaisukh Sophasan 于 2017 年在同一地点拍摄的视频清楚地显示了一只雄性Ideopsis vitrea vitrea用它的长鼻骚扰、抓挠和探测成年I. blanchardii blanchardii的翅膀。

附近达那因蝴蝶的种群密度也可能有助于更频繁地观察这种以前未知的现象。苏拉威西岛 Tangkoko Batuangus 自然保护区的沿海栖息地被茂密的沿海植被所包围，其中充满了夹竹桃科。Danaini 种是昼夜鳞翅目的优势群，特别是Danaus、Euploea、Idea、Ideopsis、Parantica和Tirumala 属. 另一种假设是，蝴蝶会被毛毛虫喂食留下的受损叶子所吸引，因此它们会靠近毛毛虫，而毛毛虫会不经意间刮伤树叶。受伤的毛毛虫及其富含生物碱的渗出液充当灯塔，引起附近蝴蝶的注意。在这个热带栖息地中不断重叠的世代也可能让新封闭的成年蝴蝶观察和学习年长个体的幼虫抓挠行为，从而导致这种不寻常行为的文化传播（Whiten 2019）。尽管我们报告了 11 种物种的出现，其中 7 种被观察到以毛毛虫为食，但随着更多的观察，这个列表很可能会增加。

据我们所知，这可能是第一次观察到马利筋蝴蝶显然是从生活的、密切相关的物种的毛虫中获取生物碱，而不是从植物或不同目昆虫的尸体中获取生物碱。我们假设生物碱从幼虫转移到成虫，但这个假设需要化学验证，因此我们谨慎解释我们的观察结果。由于幼虫似乎不愿意参与这种假定的交换，我们建议将这种行为称为“盗药行为”，这突出表明成虫消耗的化学物质是从其他人那里偷来的。不过，由于尚不清楚毛毛虫的伤口和死亡是否是由抓挠引起的，替代新词“kairopharmacophagy”（以通过“窃听”检测到的受伤毛毛虫的防御性化学物质为食）或“necropharmacophagy”（以死亡毛虫的防御性化学物质为食）也可能是合适的。目前尚不清楚这种行为是否发生在热带其他地方，还是真正仅限于该地区。例如，美洲的Danaus物种显然不知道会刮树叶（Ramos et al. 2020 , Lawson et al. 2021）。苏拉威西岛缺乏自然历史观察结果可能导致无法在岛上其他地方进行观察（但请参阅 Tea 等人，2020 年）。