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NUMBER OF INSTARS OF LARVAE OF THE ALFALFA LEAFCUTTER BEE, MEGACHILE ROTUNDATA (F.) (HYMENOPTERA: MEGACHILIDAE)

Published online by Cambridge University Press:  31 May 2012

G.H. Whitfield
Affiliation:
Agriculture Canada Research Station, Lethbridge, Alberta, Canada T1J 4B1
K.W. Richards
Affiliation:
Agriculture Canada Research Station, Lethbridge, Alberta, Canada T1J 4B1
T.M. Kveder
Affiliation:
Agriculture Canada Research Station, Lethbridge, Alberta, Canada T1J 4B1

Abstract

The number of instars of larvae of the alfalfa leafcutter bee, Megachile rotundata (F.), was determined by direct observation, frequency distribution plots, and linear regression of head-capsule measurements. For all three methods, developmental polymorphism was observed; larvae were grouped according to the number of instars. The majority of larvae (77%) had four larval instars and the remainder had five. Mortality of bee larvae, apparently a result of physical injury during measurement of head-capsule width, was high but results were based on data for 259 individuals that completed development to the prepupal (= pharate pupa) stage. Direct observation of larval moults was found to be the only accurate method for determining instar designation but analysis of head-capsule width did describe the number of instars present in a population and provided estimates of head-capsule width for each instar. A morphological key for separation among four instars is presented as a practical alternative for instar determination in field studies.

Résumé

Le nombre de stades larvaires de la mégachile de la luzerne, Megachile rotundata (F.), a été établi par observation directe, par courbes de distribution de fréquence et par régression linéaire des mesures de la capsule céphalique. Les trois méthodes ont mis en évidence un polymorphisme embryologique. Les larves ont été groupées selon le nombre d’instars. La majorité (77%) en avaient quatre, le restent cinq. La mortalité des larves, apparemment à cause de blessures infligées lors des mensurations de la capsule céphalique, était élevée, mais les résultats ont été basés sur les données provenant de 259 insectes qui ont atteint le stade prépupal (= pharate). L’observation directe des mues larvaires s’est révélée la seule méthode exacte d’identification de l’instar mais l’analyse de la largeur de la capsule céphalique décrivait bien le nombre d’instars présents dans une population en plus de fournir des estimées de ce caractère pour chaque stade larvaire. Une clé morphologique distinguant quatre instars est proposée comme solution pratique d’identification de l’instar dans les études sur le terrain.

Type
Articles
Copyright
Copyright © Entomological Society of Canada 1987

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