How-To-Do-It Use of Cobra Lily (Darlingtoniacalifornica) & Drosophila for Investgating Predator-PreyRelationships Carl R. Pratf Downloaded from http://online.ucpress.edu/abt/article-pdf/56/1/38/46417/4449739.pdf by guest on 25 September 2021 In order to investigate predator-prey tracts a variety of prey items depend- relationships in a laboratory, I have ing upon its location and the season. found the use of the cobra lily (Darling- Prey items include: ants, grasshop- tonia californica) and fruit flies (Dros- pers, flies, moths and spiders (Juniper ophila virilis) an excellent experimental et. al. 1989). system. The experiments described The cobra lily-fruit fly system is an deal with the feeding mechanism of a excellent choice for investigating pred- carnivorous plant and allow students atory-prey relationships in the labora- to examine the impact of both predator tory for a number of reasons. The and prey characteristics on the capture materials are relatively inexpensive success of the predator. The approach - and readily available to most schools. described here can be used as a labo- Both the insects and the plants require ratory exercise or the subject for an only minor care and are easy to grow independent study/research project. and maintain in the laboratory. In ad- Cobra lily is a carnivorous plant re- dition, many students have an almost stricted in range to northern California innate interest in exotic plants, and and southern Oregon (Schnell 1976; carnivorous plants in particular. The Juniper et. al. 1989), but is readily cobra lily with its unusual name and available through biological supply unique form is no exception. Other houses (e.g. Carolina Biological Sup- reasons for using this system are its ply Co., 2700 York Rd., Burlington, versatility in investigating a number of NC). The cobra lily traps insects in a different aspects of the predatory-prey tubular, hooded leaf that curves in relationship, the ease with which stu- such a fashion as to resemble the head dents can obtain sufficient data for of a striking cobra (Figure 1). The meaningful statistical analysis, use of leaves have small oval-shaped open- minimal laboratory equipment and its ings on the underside of the curved Figure 1. Close-up of Cobra lily leaf use in merging concepts of plant and hood through which insects enter the showing tongue-like appendage and animal biology. trap. Extending below the opening is a the opening through which insects en- forked appendage which is covered ter trap. Note also the fenestra (see Laboratory Procedure with nectaries, presumably for attract- text) which appear as light specks on ing prey. Insects can be observed to top of curved hood. Drosophilavirilis is cultured in a fash- land on or crawl onto the "tongue- ion familiar to genetics students using like" structure and then enter the trap. prepared instant media and virtually Once in the trap, escape is impeded by any size bottles or containers. Dros- two mechanisms. In the upper, curved that permit light to enter the inside of ophila virilis was used as the prey item region of the leaf opposite the en- the leaf, much like a window, and in our experiments because they are trance hole, there are areas that lack serve as light sources to confuse po- somewhat larger than D. melanogaster, chlorophyll and are known as areolae tential escaping prey by directing but there is no reason other insects or fenestra. These areas are regions them away from the entrance hole. could not be used if they are available. The other anti-escape mechanism is Cobra lilies do not have extensive the downward pointing hairs which root systems and are readily grown in form a Carl R. Pratt, Ph.D. is Chairman and dense lining on the inner sur- individual 9-inch plastic pots filled Associate Professor of Biology at the Col- face of the leaf tube. These hairs ap- with a mixture of sand and peat moss. lege of New Rochelle in New Rochelle, pear to impede insects from climbing The soil mixture should be kept moist NY 10805. up and out of the leaf tube. Under at all times. Additional plant care in- natural conditions, D. californica at- formation can be found in Schnell 38 THEAMERICAN BIOLOGY TEACHER, VOLUME 56, NO. 1, JANUARY1994 1990) and some introductorybiology cess as a function of prey density. The texts (e.g. Campbell1990) provide suf- use of a 3-liter bottle to form a dome ficient background in the concepts of over the entire plant is simple and predator and prey systems, in partic- effective for examining the capture ular, the role of prey density in pred- success of entire plants, but does not ator success. allow examination of individual We use two different experimental leaves. In addition, students may en- arrangements, depending upon the counter some difficulty in accounting experiment and the hypothesis under for all the flies released within the investigation. The first experimental dome. setup employs a plastic 3-liter soda An improved experimentaldesign is container with the bottom cut off to shown in Figure 3. In this setup, indi- form a dome over the entire plant vidual leaves are enclosed within lab- (Figure2). The cut bottom of the bottle oratoryflasks. The size of the flask can is pressed into the soil surface to pre- vary, but 250-mlflasks appear to work vent flies from escaping. The bottle best. The leaf is carefullyinserted into cap is removed and replaced with a the flask, the mouth of the flask is foam sponge or non-absorbentcotton closed with a foam plug through plug. The plug prevents flies from which a hole has been bored, and the escaping, allows ventilation during an plug is cut to allow it to be slipped Downloaded from http://online.ucpress.edu/abt/article-pdf/56/1/38/46417/4449739.pdf by guest on 25 September 2021 experimental trial and provides a around the base of the leaf. Alterna- means for introducingan anesthetic to tively, non-absorbent cotton may be ~~~~~~~~~I,, the flies at the end of a trial. Flies are used to form a plug in the mouth of etherized and counted; known densi- the flask and around the leaf base. The ties of flies are introduced into the flask is held in place with a laboratory dome. Anesthetized flies are intro- clamp and appropriatesupport stand. duced into the dome on a piece of This experimental setup offers an ad- white paper placed on the substrate vantage over use of the 3-liter bottles surface. After an appropriate trial because uncaptured flies are readily length, usually 24 hours, the flies not visible through the wall of the flask, in trapped by the plants and thus still many cases eliminating the necessity Figure 2. Experimental setup using visible within the dome are counted, to etherize flies remaining in the flask 3-liter plastic container to enclose en- etherized and re-counted. Flies are at the end of an experimental trial. tire plant. etherized by placing large cotton Accuracy of data collection is in- swabs saturated with ether into the creased when the flasks are used be- dome through the top opening. For cause all flies not visible within the (1976). Plants at the College of New those who prefer not to use ether flask can be assumed to have been Rochelle are maintained in a green- because of obvious hazards, products captured since there are no escape house between experiments, but there such as Fly-Nap (CarolinaBiological) routes and no hiding places. The use is no reason a sunny windowsill should also work. By introducing dif- of flasks also allows examination of would not work equally as well. Dur- ferent densities of flies into the dome, capture rates of individual leaves. ing experimental trials when more one can investigate the capture suc- Thereforeone may examine the role of controlled environmental conditions are desired, the plants are placed on a laboratory bench under fluorescent growth lights connected to a timer to provide a 12-hour photoperiod. Length of experimental trials varies, but most often a 24-hour period is used. After an experimental trial is complete, the plants are returned to the greenhouse for recuperation. In order to provide background, stimulate interest and establish an in- k~~~~~~~~~~~~~~j tellectual framework for initiating ex- periments, assign a number of read- ings in the area of carnivorous plants and predator-prey systems. Heslop- Harrison(1978) provides a good intro- duction to the carnivorous habit in plants. Articles dealing with feeding behavior of the pitcher plant (Sarrace- nia) by Cresswell (1991), Wolfe (1981) and Plummer and Kethley (1964) will provide sufficientinformation to stim- ulate interest and help generate possi- ble hypotheses for testing. Most gen- Figure 3. Flask enclosure for an individual leaf. eral ecology textbooks (e.g. Smith PREDATOR-PREYRElATIONSHIPS 39 individual leaf characteristics(such as Suggested Experiments portance in prey attraction, or cover- leaf age, size of opening, condition of ing the fenestra on the leaf hood to tongue-like appendage) on the success A number of investigations are pos- determine its importance in deterring of capture. Another important advan- sible with this experimental system. prey escape. tage to using flasks on individual The variety of hypotheses to be tested leaves is that several replicates of the is limited only by the creativity of the same experimental trial may be per- investigators. I will provide a sample Summary same of some of the hypotheses we have formed simultaneously on the experimentalsetup described in plant or, if plant material is in short investigated or plan to investigate. The The major thrust of most investiga- this articleallows for hands-on experi- supply, students may share plants and an importanteco- their experimental trials.